Hi staticx-
What directories are the 64-bit and 32-bit versions of UltraChannel located in? What directories do you have Reaper/your other DAWs looking at for 3rd party VSTs?
The 32-bit and 64-bit version of UC should be in different directories, if they are in the same one that is likely why Reaper is getting confused/not working.
As far as uninstalling/reinstalling, running the installer again will replace/update any necessary files, so you shouldn't have to worry about uninstalling first.
Best,
Tom
