I'm not sure this "fixable" on Ubuntu with the standard build of
openconnect, at least not by messing with system default priorities for
gnutls. Correct me if I'm wrong but I've done some digging this morning
and comparing the openconnect build on ubuntu 19.10 against the fedora
build the main difference with regards to the priority strings is that
the fedora build is specifically checking for a system or openconnect
default policy:

@OPENCONNECT,SYSTEM:%COMPAT

Which I believe allows you to override via system level policies for the
priority string, hence the update-crypto-policies noted in the link
above. On Ubuntu 19.10, this is the policy string I see in
libopenconnect.so.5.5.0:

NORMAL:-VERS-SSL3.0:%COMPAT

If it had a similar policy string, for example @SYSTEM or @OPENCONNECT,
you could theoretically (I haven't tested) override OpenConnect's
default using /etc/gnutls/config. I tested this priority string, which
is what Fedora sets when enabling legacy crypto, and gnutls-cli does not
complain when connecting to the AnyConnect host I have this issue with.

$ cat /etc/gnutls/config 
[priorities]
SYSTEM=NORMAL:+3DES-CBC:+ARCFOUR-128

$ gnutls-cli --priority @SYSTEM --list
Cipher suites for @SYSTEM
TLS_AES_256_GCM_SHA384                                  0x13, 0x02      TLS1.3
TLS_CHACHA20_POLY1305_SHA256                            0x13, 0x03      TLS1.3
TLS_AES_128_GCM_SHA256                                  0x13, 0x01      TLS1.3
TLS_AES_128_CCM_SHA256                                  0x13, 0x04      TLS1.3
TLS_ECDHE_ECDSA_AES_256_GCM_SHA384                      0xc0, 0x2c      TLS1.2
TLS_ECDHE_ECDSA_CHACHA20_POLY1305                       0xcc, 0xa9      TLS1.2
TLS_ECDHE_ECDSA_AES_256_CCM                             0xc0, 0xad      TLS1.2
TLS_ECDHE_ECDSA_AES_256_CBC_SHA1                        0xc0, 0x0a      TLS1.0
TLS_ECDHE_ECDSA_AES_128_GCM_SHA256                      0xc0, 0x2b      TLS1.2
TLS_ECDHE_ECDSA_AES_128_CCM                             0xc0, 0xac      TLS1.2
TLS_ECDHE_ECDSA_AES_128_CBC_SHA1                        0xc0, 0x09      TLS1.0
TLS_ECDHE_ECDSA_3DES_EDE_CBC_SHA1                       0xc0, 0x08      TLS1.0
TLS_ECDHE_ECDSA_ARCFOUR_128_SHA1                        0xc0, 0x07      TLS1.0
TLS_ECDHE_RSA_AES_256_GCM_SHA384                        0xc0, 0x30      TLS1.2
TLS_ECDHE_RSA_CHACHA20_POLY1305                         0xcc, 0xa8      TLS1.2
TLS_ECDHE_RSA_AES_256_CBC_SHA1                          0xc0, 0x14      TLS1.0
TLS_ECDHE_RSA_AES_128_GCM_SHA256                        0xc0, 0x2f      TLS1.2
TLS_ECDHE_RSA_AES_128_CBC_SHA1                          0xc0, 0x13      TLS1.0
TLS_ECDHE_RSA_3DES_EDE_CBC_SHA1                         0xc0, 0x12      TLS1.0
TLS_ECDHE_RSA_ARCFOUR_128_SHA1                          0xc0, 0x11      TLS1.0
TLS_RSA_AES_256_GCM_SHA384                              0x00, 0x9d      TLS1.2
TLS_RSA_AES_256_CCM                                     0xc0, 0x9d      TLS1.2
TLS_RSA_AES_256_CBC_SHA1                                0x00, 0x35      TLS1.0
TLS_RSA_AES_128_GCM_SHA256                              0x00, 0x9c      TLS1.2
TLS_RSA_AES_128_CCM                                     0xc0, 0x9c      TLS1.2
TLS_RSA_AES_128_CBC_SHA1                                0x00, 0x2f      TLS1.0
TLS_RSA_3DES_EDE_CBC_SHA1                               0x00, 0x0a      TLS1.0
TLS_RSA_ARCFOUR_128_SHA1                                0x00, 0x05      TLS1.0
TLS_DHE_RSA_AES_256_GCM_SHA384                          0x00, 0x9f      TLS1.2
TLS_DHE_RSA_CHACHA20_POLY1305                           0xcc, 0xaa      TLS1.2
TLS_DHE_RSA_AES_256_CCM                                 0xc0, 0x9f      TLS1.2
TLS_DHE_RSA_AES_256_CBC_SHA1                            0x00, 0x39      TLS1.0
TLS_DHE_RSA_AES_128_GCM_SHA256                          0x00, 0x9e      TLS1.2
TLS_DHE_RSA_AES_128_CCM                                 0xc0, 0x9e      TLS1.2
TLS_DHE_RSA_AES_128_CBC_SHA1                            0x00, 0x33      TLS1.0
TLS_DHE_RSA_3DES_EDE_CBC_SHA1                           0x00, 0x16      TLS1.0

Protocols: VERS-TLS1.3, VERS-TLS1.2, VERS-TLS1.1, VERS-TLS1.0, VERS-DTLS1.2, 
VERS-DTLS1.0
Ciphers: AES-256-GCM, CHACHA20-POLY1305, AES-256-CCM, AES-256-CBC, AES-128-GCM, 
AES-128-CCM, AES-128-CBC, 3DES-CBC, ARCFOUR-128
MACs: SHA1, AEAD
Key Exchange Algorithms: ECDHE-ECDSA, ECDHE-RSA, RSA, DHE-RSA
Groups: GROUP-SECP256R1, GROUP-SECP384R1, GROUP-SECP521R1, GROUP-X25519, 
GROUP-FFDHE2048, GROUP-FFDHE3072, GROUP-FFDHE4096, GROUP-FFDHE6144, 
GROUP-FFDHE8192
PK-signatures: SIGN-RSA-SHA256, SIGN-RSA-PSS-SHA256, SIGN-RSA-PSS-RSAE-SHA256, 
SIGN-ECDSA-SHA256, SIGN-ECDSA-SECP256R1-SHA256, SIGN-EdDSA-Ed25519, 
SIGN-RSA-SHA384, SIGN-RSA-PSS-SHA384, SIGN-RSA-PSS-RSAE-SHA384, 
SIGN-ECDSA-SHA384, SIGN-ECDSA-SECP384R1-SHA384, SIGN-RSA-SHA512, 
SIGN-RSA-PSS-SHA512, SIGN-RSA-PSS-RSAE-SHA512, SIGN-ECDSA-SHA512, 
SIGN-ECDSA-SECP521R1-SHA512, SIGN-RSA-SHA1, SIGN-ECDSA-SHA1

$ gnutls-cli --priority @SYSTEM your-vpn-host.tld

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https://bugs.launchpad.net/bugs/1822467

Title:
  OpeonConnect fails with generic TLS Fatal Alert Error

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