Session hanging with TM REQUEST and plan handle 0x0000
TM stands for Transaction Manager. The current command being set to TM
Request means that the thread is currently servicing a DTC (Distributed Transaction Coordinator) request, either to enlist in a DTC transaction, or defect from it, or commit it, or so on. Distributed Transactions are not supported for Database Mirroring because it can't guarantee transaction integrity.  Suppress your DTC transactions and this should go away.