Mantis Bug Tracker - Forums

So the past week or so I have tried updating Mantis from 1.1.0a4 to 1.3.xxx (I have since reverted to a snapshot, due to lots of customizations not being there). The main reason I wanted to update was due to application error 700 project not found, this occurs when anybody logs into mantis. Any tips would be greatly appreciated to help me resolve this issue.

That is a tough question, could also be caused by one of the customizations.
Still would do the upgrade and review all customizations and replace with plugins for as far is possible.