Problem with vss backup of a specific SQL instance

Hello,

On a windows 2008 r2 standard server, we host 2 SQL 2008 r2 enterprise instances with sp1 installed.  One of them, the named instance,  contains 500-600 databases and the default instance hosts only one. We only have C drive and the ext backup disk. The windows server backup fails with a vss error and the vssadmin lists writer sqlserverwriter as inconsistent shadow copy. If i restart the sql vss writer and stop the named instance service, the backup starts fine with the named instance service working. If I start the backup with both instances running and sql vss writer stopped, the backup also runs ok. Should I find a way to have the backup running with both instances and the sql vss writer service started? I succesfully restored an mdf and ldf file from the backup.