Problem solved by eliminating Surena.
Before reading this wiki article indeed similar things have happened in our pbem game:
This, however, could easily be Roman propaganda. Orodes II, with the rest of the Parthian Army, defeated the Armenians and captured their country. However, Surena's victory invoked the jealousy of the Parthian king, and he ordered Surena's execution. Following Surena's death, Orodes II himself took command of the Parthian army and led an unsuccessful military campaign into Syria. The Battle of Carrhae was one of the first major battles between the Romans and Parthians.