Spent part of a day chasing down an SQL Server Always On Availability Group manual failover, failure issue.
When running the failover wizard, the failure looks like this:
Some of the error description: Manual failover failed. Failed to perform a manual failover of the availability group ‘x’ to server instance ‘y’. Error code 5015.
The Availability Group and the Listener Name were named identically. That was causing the failure, to Windows Cluster, it was a name conflict as both resources could not be shared.
Renamed the AG name, problem solved!