I was getting this very vague error message on a new installation of BuddyPress when attempting to create new Forum topics – There was an error when creating the topic.
After trying various proposed fixes I realised that my Groups had been created before the Forum was installed so I checked the database and there were no forums relating to my groups on the table wp_bb_forums. The solution is simple! Just go to group page > Admin > Group Settings and uncheck the option “enable discussion forum” the save changes then check “enable discussion forum” and save the changes again. This will create a Forum entry on the database for that specific Group.
Hope this helps anyone struggling with this same error.