Thanks for the detailed information. The mutex isn't necessarily the source of the problem: it is only used to detect another instance. For communication between the existent and the new instance a TCP connection is used on port 49876 on 127.0.0.1. Over this TCP connection the new instance sends the filename or other actions to the existent instance.
Maybe you set up a firewall or anything else caused the TCP connection to break?
I'll check if we can add some more useful debug/error output in the relevant code.