Hi BDAqua, thanks for your help so far.
An interesting development ~
1) Safe Boot : There is no error & the drive does not auto-mount. In "login Items" there is an exclamation mark next to the drive.
I can select the drive manually in Shared side bar & it then mounts without error.
2) After that I rebooted into my usual account - The error appeared but this time no drive mounted after clicking OK. That's different from before. It also shows an exclamation in Login Items. I was able to connect manually.
3) I created a new Admin user & re-booted. This time I had Automatic Login disabled. I decided to login to my normal account just to check. This time the drive mounted correctly without an error.
4) I re-enabled Auto-login & rebooted - again error pops up but no drive mounts after 'OK'.
So it looks like Auto-login OFF fixes the issue, but I wonder if there is a way to fix it with Auto-login ON?
Also something has changed in the behaviours after safe boot, which seems odd.
Note: I have not tried the new admin account yet due to the above result.