On my B2601 the rules are:
1. If sitting in the seat, most things are fine
2. If off the seat, then you need:
- PTO lever disengaged. Maybe the PTO switch is faulty
- HST pedal perfectly centred. Sometimes these get sloppy, if you nudge the HST pedal does it fix it?
3. If off the seat and wanting to use the PTO, you need the seat flipped up. The switch for this is on the front of the seat pedestal, and is fussy on mine, sometimes needs a jiggle
If I was guessing, I'd say chances are that one of your PTO switch or your HST switch is not working properly. They also stop the machine from starting, so when the machine stops, you can try starting (shouldn't work), then jiggle one, try starting, then jiggle the other, try starting. That will tell you which one is the one that's giving issues.