I just upgraded to MT3.11 and something went wrong. I still don't know what, but if you try to add a comment, it won't work. I can't fix it right now, so please be patient and go out somewhere (it's the week-end, you shouldn't be here ;-).
Update: everything seems to work fine now. I had numerous FTP problems during the whole week-end which screwed up files at random. The upgrade did go fine, apparently, but nothing worked. I had to upload a full version on top of the MT installation, but each rebuild caused a Can't call method "id" on an undefined value at lib/MT.pm line 770 error. Six Apart support was responsive and helpful, their diagnosis steps helped me figure out by myself that I stupidly forgot to re-upload the plugins. No comment has been lost, I'm rebuilding those entries manually. Still, I wish MT upgrades were simpler.
I have the same error but only when I turn on "Monthly Archive". I upgraded to 3.1 (haven't gone to 3.11 yet) and I agree.. wish upgrades were easier. I lost so much of my customizing in my build files. ARGH. It's somewhat stable for the moment (if I turn off monthly archiving) so am afraid to "re-upgrade".
Any suggestions?
It might be that you're using a plugin within that template that either is not in your current installation, or is not compatible with the version you're using.
I found it. My default date based template had something weird in the "allow comments" area. *Shrug* It wasn't an issue before. I guess 3.1 didn't like it.
Thanks!