As far as I understand, you have been runing a physical stand-by. That means that both databases have same DBID and they are physically identical, what means they have same dbid and the RMAN cannot differentiate the stand-by database from the primary one. I have the feeling that you have registered into the recovery catalog your stand-by database but not the original one. Now when your stand-by is down, the RMAN still keeps up resync from the stand-by control file, which however is not advanced forward anymore, since the stand-by is down and that causes the error.
I can't really imagine how that could happen but seems that is the case. Maybe it worths to try changing the snapshot controlfile name and try again
set snapshot controlfile name to 'something'