I was getting the following error “Invalid domain name. Domain name is either invalid or unreachable” while trying to register a plugin.
Found this kb article
So installed update rollup 4.0
After that on trying to open CRM started getting the could not find GUID error.
Not very sure, but it has something to do with ActiveDirectory i guess.
Anyways uninstalled the Rollup 4 and to my surprise everything started working normally, even i was able to register plugin without any issues!!
Just thought of sharing the same !!