Webroot's Identity Shield module can blacklist Mailbird by setting it to Deny in its Protected Applications dialog. This can cause copy/paste not to work, or worse, cause Mailbird to crash when pasting into it. Changing Mailbird to Protect (and if necessary, to Allow) should stop all of the pasting issues in Mailbird.

Note you may have to do this again each time you update Mailbird, until Webroot implements file location based identification of programs, which I'm told is on its way.