this error seems refers to private message plugin, that's not compatible with MyPMS OperSource 2.5 component. This bug was submitted by myself, and fix was easy.
Anyway I suggest you to try a better solution that MyPMS because it seems an inactive project and still suffer many bugs.
Try to install UddeIM instead. It's the private message system of this site now. You can also find a forum where talk about UddeIM component.