Hello,
Sorry to hear you are having trouble.
I don't know how this was missed but my apologies for it.
I can change the encryption key in the Secondary without issue, but I cannot remove it.
But DID you change it? If so, and instead of trying to change it back, burn it and reinstall on the Secondary:
https://forums.stardock.com/486104/multiplicity-support-faq#reinstalling
If you still have the issue, is either PC you are installing on NOT under a Windows Administrator account? If so, and if possible, at least for the install, do so under an admin account.
Again, sorry for the delay.
Sean Drohan
Stardock Support Manager