Comment 3 for bug 418853

What's happening here is that there's a running instance of the Transmission GTK+ client somewhere, but it's not responding. The new instance is asking the first one to pop up and show itself to the user. This request is made via dbus, and the error message the OP is seeing is from that low-level failure.

I've added an error popup to help explain the situation, and its resolution, to end users:
http://trac.transmissionbt.com/attachment/ticket/2387/lockfile-err.jpg

However, this only addresses the symptom... not the question of why there's a unresponsive copy of Transmission lying around somewhere.

If this is repeatable, Alex, could you please use gdb to attach to the unresponsive version of Transmission to see what it's doing?
To do this, there are three steps: (1) find the transmission pid, such as through ps or the Gnome System Monitor (2) attach to it with gdb: "% gdb attach $pid" (3) get a backtrace in gdb: "(gdb) thread apply all bt"

If you want more help doing this, I'm happy to help.