For other newbs out there....
Enabled debugging in Administration -> Development -> Debugging
Which showed me that I had a username with invalid characters (but couldn't see who it was).
So I edited the /user/lib.php file according to this https://moodle.org/mod/forum/discuss.php?d=244011
This bypassed the error so I could see who it was.... *somehow* they had a space in their samaccountname.
Removed the space - undid the lib.php edits and all is good.