After tracing sane-plustek in gdb, I found that it tries to create the lock file /var/lock/sane/LCK..auto - but without creating /var/lock/sane/ first.
I assume that at some point, this directory was created when installing the package, but this is no longer valid with /var/lock pointing to /run/lock. That directory can be on a ramdisk and is always cleared out at boot. Please ensure that the directory is always created before writing the lock file there.
