Might not be related to the EmailReporting plugin specifically, I've been playing in the plugins directory and so I might have borked something else up....GRRRRRR.
EDIT: it looks like it might be a different plugin that is incomplete and when I hit the [Install] button it broke Mantis. Blah. Now I need to figure out how to manually uninstall that plugin since Mantis is unusable now.
Just to be sure i double checked against the latest MantisBT and EmailReporting versions. Both worked without issues
Concerning your issue i would try the following
Open your sql editor (eg. PHPmyAdmin)
Go to the table mantis_plugin_table
Modify "enabled" to "0" for the offending plugin