Running 5.0.3.2622 RC1 and in options associated files with MM. Com Elevator ran and Win 10 default app page loaded. Set default players. No problems. File exit MM. Installed 5.0.3.2625 RC2 . Would not install because MM still running. Apparently exiting MM did not close the elevator on exit. Ran task manager to close MediaMonkeyCOM.exe. Retry worked. Installed fine.
Installed version RC2 and used associate files in options on a different machine to verify RC2 elevator issue the same as RC1. Same effect after installing This only occurs the first time you associate files after an installation.

Thanks,
signman3