I am getting this error in RC6.

PM sends OK, but when a member attempts to reply, that is when the error occurs.

Sorry, there is a problem

Something went wrong. Please try again.

Error code: EX2

 

Messages are posted, however they are duplicated.

If anyone else encounters this problem, the culprit was a corrup logo in ACP> System> Email Settings> Email Logo - The size was reported as 0kb

Deleted the logo and reuploaded, messages working as expected.

Thanks to Alan in support.