During roles removal I encountered this error on the schema master with the command regsrvr32 schmmgmt.dll :

The Module "schmmgmt.dll" Loaded but the Call to DllRegisterServer Failed with Error Code 0x80040201

Solution:

Make sure the computer account belongs to "schema admins" otherwise you won't be able to do this operation, then run the CMD in administrator mode, and it will take care of the issue.

Another solution, if you would have been stuck without resolving this issue, is to use the NTDSUTIL command to manually transfer the FSMO to another computer.