The issue of sp_MScheckIsPubOfSub stored procedure
Please check for orphaned rows for deleted subscriptions in sysmergesubscriptions and delete them.  For example, the subscriber that this is failing on may have duplicate entries in sysmergesubscriptions, one being orphaned from a previously deleted subscription.

This has been the cause of this error and solution for clients of mine in the past, ymmv.  I hope this helps.