Sorry but I can not believe a short in the horn would cause this. If it was a dead short then first of all it would not work and secondly either a fuse would blow or the switch and wiring to it would melt. I doubt it a good battery goes on its knees enough for a ECU to suffer before the wiring to the horn melts ... how would you expect 1.5mm2 to handle the curent from something like a starter motor?
The horn functioning migh generate enough spikes on the supply for the ECU to get confused, esspecially if the signal from another sender is too low though (flywheel?). Check for ground and cable connections on those I would say ...
My own 2P worth .....