I've had this happen at times and it can be frustrating. Sometimes rebooting and trying to add the add-in works but sometimes not. You can also change the GUID for the application (shown as the Class ID in the form) and recompile it. That way EPDM sees it as a different add-in if it somehow has been tricked into thinking it is still loaded.
You can restart archive server then try to add the addin in the debug mode will solve the issue.
I had the exact same problem.
After some research with the support, the solution was tricky but easy :
Administrative Tool > Your user account personal settings (not Admin) > Administrative Permissions > Can administrate add-ins.
I had to do this even if I am logged as Admin in the Administrative Tool for Add-In management.
I hope it'll help.