If I force the SMB1 protocol (vers=1.0), then the mount works:
root@ubuntu:~# mount //localhost/pub /mnt -o guest,vers=1.0 root@ubuntu:~# mount -t cifs //localhost/pub on /mnt type cifs (rw,relatime,vers=1.0,sec=none,cache=strict,uid=0,noforceuid,gid=0,noforcegid,addr=127.0.0.1,soft,unix,posixpaths,serverino,mapposix,acl,rsize=1048576,wsize=65536,echo_interval=60,actimeo=1) root@ubuntu:~# uname -r 5.0.0-7-generic root@ubuntu:~#
If I force the SMB1 protocol (vers=1.0), then the mount works:
root@ubuntu:~# mount //localhost/pub /mnt -o guest,vers=1.0 vers=1. 0,sec=none, cache=strict, uid=0,noforceui d,gid=0, noforcegid, addr=127. 0.0.1,soft, unix,posixpaths ,serverino, mapposix, acl,rsize= 1048576, wsize=65536, echo_interval= 60,actimeo= 1)
root@ubuntu:~# mount -t cifs
//localhost/pub on /mnt type cifs (rw,relatime,
root@ubuntu:~# uname -r
5.0.0-7-generic
root@ubuntu:~#