wpmu 2.7.1 and SimplePress forum

Hey there.
2 days ago I noticed my forums were no longer working, the site stopped in the middle of the page generation. So the source code of the page containing the forum is cut off, see here: http://zice.ro/forums/

I checked my php logs and error logs and here is what I find:

Fatal error: Call to undefined function sf_render_page() in /mydomain/wp-content/plugins/simple-forum/sf-control.php on line 306

also see my posts on the simpel press forum:
- http://simplepressforum.com/support-forum/simplepress-forum-on-mu/forum-stopped-working/
and a related post: http://simplepressforum.com/support-forum/spf-version-4/forum-version-40-fatal-error/

unfortunately I can't retrace the changes since it last worked (last week or so?) as there were too many plugin upgrades in between and disabling all plugins would be a chore, is there any chance someone could help tracing this? I mean when it comes to generate the forum, there seems to be a clash with another plugin... but I am unsure how or what else to do :-9