Ubuntu Server 18.04.01 LTS CIFS Mount failing

Remove sec=ntlm. That hasn't worked for a few versions now, but it's still in lots of tutorials around the interwebs.

It's not that sec=ntlm no longer works. That setting is inconsistent with the version of the samba dialect you are using however.

I have an Ubuntu 18.04 Samba server set up. If I try to mount with your settings:

sudo mount -t cifs //gort.local/Public /home/morbius/Test -o guest,vers=3.0,sec=ntlm

It fails with the exact "Invalid argument" error message and logs the exact same errors.

If instead I run the mount command with a version of the smb dialect consistent with that security setting:

sudo mount -t cifs //gort.local/Public /home/morbius/Test -o guest,vers=1.0,sec=ntlm

It does work:

mount | grep cifs

//gort.local/Public on /home/morbius/Test type cifs (rw,relatime,vers=1.0,sec=ntlm,cache=strict ....

If you want to connect to the server with sec=ntlm change to vers=1.0 - I don't know why you would want or need to do that but .....

Or keep vers=3.0 and drop sec=ntlm in which case sec defaults to ntlmssp which is consistant with SMBv3.