One other possibility - there are two means by which the camera determines that a lens is fitted. One is the obvious EF contacts along the bottom of the lens mount, the other is a much less obvious microswitch behind the front layer of the mount itself, located just above 9 o'clock looking from the front of the camera.
If this switch is tripped, the camera knows a lens is fitted and tries to communicate with it. A T-mount will, of course, not reply so the camera assumes an error and report s it as a lens communication error - possibly the same error as you are getting.
The point is that some T-mounts don't take this into account and the relevant part of the bayonet is long enough to trip that microswitch. The solution, if this is the case, is to take a file to the T-mount and shorten that lug of the bayonet fitting - about 1/8" should be sufficient.
HTH
Chris