I finally found the problem with REX 1524, using the upgraded version of Salzburg-Schwarzach St. Veit, AI crash just before St. Johan in Pongau. Both problems were at at Bischofshofen. AI trains first crashed into a cargo consist, that seemed out of place as it was over a switch, so I deleted it. But got a new crash, this time a little closer to St. Johan. I observed that the AI now crashed in the Railjet consist at Bischhofshofen. So I moved that consist to the next track. Which finally made it start it's journey towards Salzburg, solving the problem permanently.
Did you make this scenario for the original scenery? I guess upgrading it with ØBB signals could make trouble then? Another theory I have is that on some computers, consists can "pop" into a track they are not supposed to be in. Would be interesting to hear your opinion of this, and if you want to do something to REX 1524, I can help with pics from Bischofshofen before and after fixing.