Reproduced the steps as indicated by Bernhard. I got the same results as Bernhard with the backend "test". For the actual scanner I use the backend plustek. scanimage -L lists the correct scanner. Executing the actual scan gives an error:
scanimage: open of device net:localhost:plustek:0 failed: Error during device I/O
aug 21 12:33:17 eaze systemd[1]: Started Scanner Service (127.0.0.1:34338). aug 21 12:33:17 eaze audit[1]: SERVICE_START pid=1 uid=0 auid=4294967295 ses=4294967295 subj==unconfined msg='unit=saned@8-127.0.0.1:6566-127.0.0.1:34338 comm="systemd" exe="/lib/systemd/systemd" hostname=? addr=? terminal=? res=success' aug 21 12:33:17 eaze saned[2946]: saned (AF-indep+IPv6+systemd) from sane-backends 1.0.27 starting up aug 21 12:33:17 eaze saned[2946]: check_host: access by remote host: ::ffff:127.0.0.1 aug 21 12:33:17 eaze saned[2946]: init: access granted to patrick@::ffff:127.0.0.1 aug 21 12:33:17 eaze saned[2946]: [sanei_debug] Setting debug level of dll to 255. aug 21 12:33:17 eaze saned[2946]: [dll] sane_init: SANE dll backend version 1.0.13 from sane-backends 1.0.27 aug 21 12:33:17 eaze saned[2946]: [dll] sane_init/read_dlld: attempting to open directory `/etc/sane.d/dll.d' aug 21 12:33:17 eaze saned[2946]: [dll] sane_init/read_dlld: using config directory `/etc/sane.d/dll.d' aug 21 12:33:17 eaze saned[2946]: [dll] sane_init/read_dlld: considering /etc/sane.d/dll.d/libsane-extras aug 21 12:33:17 eaze saned[2946]: [dll] sane_init/read_config: reading dll.d/libsane-extras aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `ls5000' aug 21 12:33:17 eaze saned[2946]: [dll] sane_init/read_dlld: considering /etc/sane.d/dll.d/hplip aug 21 12:33:17 eaze saned[2946]: [dll] sane_init/read_config: reading dll.d/hplip aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `hpaio' aug 21 12:33:17 eaze saned[2946]: [dll] sane_init/read_dlld: done. aug 21 12:33:17 eaze saned[2946]: [dll] sane_init/read_config: reading dll.conf aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `net' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `abaton' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `agfafocus' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `apple' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `avision' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `artec' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `artec_eplus48u' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `as6e' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `bh' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `canon' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `canon630u' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `canon_dr' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `cardscan' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `coolscan' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `coolscan3' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `dell1600n_net' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `dmc' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `epjitsu' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `epson2' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `epsonds' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `fujitsu' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `genesys' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `gt68xx' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `hp' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `hp3900' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `hpsj5s' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `hp3500' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `hp4200' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `hp5400' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `hp5590' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `hpljm1005' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `hs2p' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `ibm' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `kodak' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `kodakaio' aug 21 12:33:17 eaze audit[1]: SERVICE_STOP pid=1 uid=0 auid=4294967295 ses=4294967295 subj==unconfined msg='unit=saned@8-127.0.0.1:6566-127.0.0.1:34338 comm="systemd" exe="/lib/systemd/systemd" hostname=? addr=? terminal=? res=success' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `kvs1025' aug 21 12:33:17 eaze systemd[1]: saned@8-127.0.0.1:6566-127.0.0.1:34338.service: Succeeded. aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `kvs20xx' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `leo' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `lexmark' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `ma1509' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `magicolor' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `matsushita' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `microtek' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `microtek2' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `mustek' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `mustek_usb' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `mustek_usb2' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `nec' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `niash' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `pie' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `pint' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `pixma' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `plustek' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `qcam' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `ricoh' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `rts8891' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `s9036' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `sceptre' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `sharp' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `sm3600' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `sm3840' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `snapscan' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `sp15c' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `tamarack' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `teco1' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `teco2' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `teco3' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `u12' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `umax' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `umax1220u' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `v4l' aug 21 12:33:17 eaze saned[2946]: [dll] add_backend: adding backend `xerox_mfp' aug 21 12:33:17 eaze saned[2946]: [dll] sane_open: trying to open `plustek:0' aug 21 12:33:17 eaze saned[2946]: [dll] load: searching backend `plustek' in `/usr/lib/x86_64-linux-gnu/sane:/usr/lib/sane' aug 21 12:33:17 eaze saned[2946]: [dll] load: trying to load `/usr/lib/x86_64-linux-gnu/sane/libsane-plustek.so.1' aug 21 12:33:17 eaze saned[2946]: [dll] load: dlopen()ing `/usr/lib/x86_64-linux-gnu/sane/libsane-plustek.so.1' aug 21 12:33:17 eaze saned[2946]: [dll] init: initializing backend `plustek' aug 21 12:33:17 eaze saned[2946]: [dll] init: backend `plustek' is version 1.0.0 aug 21 12:33:17 eaze saned[2946]: saned exiting
