As usual, I can only guess given info provided. So ... 3.0 to 3.3 skipped 3.1 and 3.2.
I know that you're still using the old method of ftp new code, move old code to .old, copy back in addons/plugins, etc.. .... maybe it's time for git where one could 'march' the site through the intermediate versions ... check for errors ... if errors present, fix them at that version, then next step in the 'march' upwards.
Those plugins you mentioned throwing errors ... previously ... and still in the upgraded version ... some of those might be biting.
So, mind if I PM you?
'spirit of sharing', Ken