I'd suggest to check why you're exploding, aka do 'limits' to 'soft'
God, of course... thanks!
Now I know, it is simply a "orbital structure exceeded" and the reason is the trajectory. For some reason, at the time of the expected MECO, the Shuttle pitches up. It actually sorts of follow the display. The problem is that if I am a bit late at MECO, it pitches up like crazy and starts looping as hell very fast. No surprise I am exploding almost instantaneously. This is happening while flying late a manual MECO. But it also occurs in full AUTO with only one engine running. This situation happens sometime if a failure scenario leads to 2 engines lock-up, I am switching throttle to manual to cut them off before too much gs build up, then move back to AUTO to get the last running engine doing his job for MECO, and bang.
PS: I am running next now, but I can do some test on 2020.* as well.
Edit: Just realized that it may be the preset ascent stage 2 in the mission file, it has indeed a kick in the last point. I am toying with it, adding a point at same altitude to see if that improves the final attitude.