From reading posts of owners of other models of tractors especially on newer year tractors, the safety system for the starter is not a simple safety system like a garden tractor which would have each safety switch wired in series with the next until the starter solenoid is reached.
Rather there is likely an electronic box called an " Operator Presence Control" which receives individual signals from each safety switch and then, based upon its programmed internal logic will either allow the tractor to start or even shut it down.
An example of this logic would be the pto lever. If the tractor is running, pto engaged, and you get off the seat, does an alarm sound or does the engine stop?
When you try and start the engine and the pto lever is engaged, the starter should not receive any power.
Can you please tell us how the pto safety switch operated previously when all was good.
The Operator presence electronic control would trigger a relay to send power to the starter. Look at the fuse and relay assembly on your tractor for something blown or loose in its socket
The following info is for a B21 which is also HST somay be useful in understanding your tractor.
Dave M7040
