As a temporary solution, hold shift while starting osu! until a recovery window pops up and disable compatibility mode.
If you are unable to start the repair dialog, navigate to your installation folder (default path is C:\Users\<Username>\AppData\Local\osu!) and edit the file "osu!.pcname.cfg". Change the value of the line "CompatibilityContext" to 0. Save the file and then attempt to open osu!. If you ever enable Compatibility mode in the future this issue will reappear.