Hey B1rd13,
I think I found a workaround for this issue where you don't need to restart your PC.
If the problem occurs open you Task Manager and enable "Show Processes from all Users". Than look for an application called action_svc.exe or so. Kill this process. Than try to start Action!. After that it should tell you you need to reinstall the software. Ignore this message and close the application. Than have again a look into the Task Manager and exit the same application if it is running. (I can't remember if it was running the second the time) Than try to start Action! once again. Now it should start normal and everything is as usual.
This worked for me. But I can't guarantee it will work for you.
Anyway, if someone of Action! is reading this you might look at your code which prevents multiple instances of Action!. For me it looks like this is a bit buggy
Best regards,
Snake