I've had this in the past. I don't remember the cause/solution, but could be a problem with reaper_rex.dll or REX Shared Library.dll. Assuming you've done the obvious like re-install and reset your computer, try installing a portable version of Reaper and see if it solves the problem. If so, copy those 2 files from the new version to the existing one. If that doesn't do it your reaper.ini file could be the problem. Rename or backup the existing one and copy the one from the portable install. If it works, then you know the cause. You'll lose all your existing settings with the fresh .ini, but they occasionally get messed up.