Just a follow up moving to r2014 fixed the problem.
I’m having the same issue it seems line 212 of functions.bb-topics.php is where things go south. I am running bbPress is 1.0-Alpha-6 and WPMU 2.7.
Effectively bbPress tries to insert an empty string into the topic_id field, which is an INT with auto increment and of course MY SQL returns in error “Incorrect integer value: ” for column ‘topic_id’ at row 1″.
Digging into the code further the defaults array is setting topic_id = false on line 151, remove topic_id from the defaults and things work fine. Looking at trac it seems this was fixed in r2014. I’m going to install and see what happens.
