This is a known bug that will be fixed in the next version. It is a intermittent bug that some users rarely encounter and others more often. Usually it occurs when Multi-Edit has been running a long time or lots of session changes have been made. A temporary work around, without restarting the editor is to use Paste Special when pasting from sources outside the editor. If you use sessions and/or projects usually a restart is quick and all your open files are reopened on restart.