OK, thanks for your suggestions.
I have tried some things you mentioned:
I use only standard and SBB route assets.
No 'strange' asset names
I had actually already started to make some new scenarios from scratch just to get rid of any possible corruptions in the existing scenarios.
The problem still was not solved (even with one loco driving to one destination...)
So next step was changing the wheather and season to clear/summer. I started with a standard scenario, just the RE4/4 with final destination after the problem area.
After editing: Save/start scenario - OK, immediately quit and restart - OK, drive scenario - OK, end scenario and restart - NOT ok!! SBHH error !? Restart from main menu - OK...
Then I added a passengercar and a passengerload stop. Still working but with above issue of SBHH error when restarting the scenario after first succesfully running it..
Then I added another passengercar and 2 more stops: Still runs OK but the SBHH error still keeps popping up now and then starting the scenario, not during the scenario however. Weird!!
Now it really gets interesting:
I changed the weather back to the original 'Wolken am horizon' (SBB route) because I thought that must have been the problem but.... It still runs OK with that weather too!!
Now I am totally confused.. I don't have a clue what has caused the problem and although I can now run scenario's without crashing halfway through it is still a bit wonky because of the (random) SBHH errors restarting the scenario...
I will probably spend some more time trail & erroring this to see if I can find the issue.
If I find anything interesting I will let you know!