The MIDI event 144 (hex 90) is note off.
The problem may be ocurring if you connect the cable with some note on sustained.
So the SQ-R receives an note off event without a previous note on.

Normaly it doesn't cause an error message on any synth, but I don't know how the SQ-R manages that.

I have an ESQ-1, which uses the same battery and it's very easy to find, as it's well used on the kodak photo cameras.

If you don't find it on conventional stores, try a look on photo stores.