I have experienced several problems with the starter relay contacts welding on my '85 K100RT. I had this re-occur several times and finally took the bike to the BMW dealer. After a month I got the bike back and they said it was a faulty ignition switch. Two days later, I welded the contacts again. Now really peeved with the dealer, I pulled out the schematic and traced down the problem.
I found the clutch interlock switch when closed had approximately 15 ohms of resistance across it. This in series with the starter relay coil caused a voltage drop across the relay coil much like a low battery voltage. It was a $16 fix. Needless to say the dealer was a little red-faced.
In a pinch, if the the contacts have welded, try rapping on the relay with a screwdriver handle several times. I've able to break the contacts loose this way.Hope this helps,