Ur44 problems with connectors, no sound, cracking when touched

A year or so ago i replaced my ur22 with a second hand ur44. Since a few months i had sudden loss of signal on the main output, on either channel. Plugging the connector(s) out and in would fix the problem. I bought a new cable but it didn’t help. One time i plugged the connector on the other end, on the amp, out and in, and to my surprise that fixed the error too.
So one time i decided it was enough and put back my ur22 again. Only to find that it too suddenly had no sound on a channel, and when touching the connectors i got a crackle and sound back again. I’ve been using the ur22 for a bit now and it has not happened again, it seems less error prone than the ur44.

What could be the error here? Some loose contact? Why did sound come back when (un)plugging the cable at the amp side?
And what can i do about that ur44, i spent some 230 euro on it and it 's a shame to throw away.

Thanks very much for any helpful reply.