Sql Server 2008 R2 - Failed to send an environment change notification to a log shipping partner node
I guessed at several things in the comments above, but the one that hit home was the connection string having the failover partner attribute. 

This could actually be what is telling SQL Server to go validate the login on the partner (and yes, I realize that is for mirroring, but this wouldn't be the first time SQL Server's error messages could be misleading).

It is possible that the login no longer has access to the database on the partner (e.g. their rights have been removed) or that the database is offline or no longer exists. Which database? Could be the login's default database (check both instances), the database specifically hard-coded into the connection string, or possibly a database mentioned in a logon trigger or classifier function.