Thanks for the input. Very interesting... So what I did so far:
First I used a new patch without any effects and just added the Effects-Loop 1+2 in parallel with the Mixer set 100% left and 100% right. With this setting it stayed the same: better/thicker sound when inverted and not normal.
So it can't be blocks that are causing it, I would say.
Then I checked the cabs.
Thanks for the link. Everything fine here (red=+ and black = -). I also used two different cabs and used all possible configurations with 4 cabs - still the same: better/thicker sound when inverted and not normal. And also: I checked every constellation with including the ping pong delay: every time in normal mode the repeats of the delay sounded distorted, thin and much too quiet (maybe you can argue about the sound in normal and inverted. Maybe it is just my ears but the ping pong delays in normal are 100% not intended). With inverted everything normal.
So maybe that is just what it is and inverted is the new "normal"?
But then again:
So still, no clue what is going on.
The one thing I learned from all this: always have one footswitch assigned to the normal/invert in every patch so that you can fastly switch if needed. OK, if you use the same setup (cabs etc.) all the time it wouldn't be necessary but if you are running into different cabs (maybe share a stage and use the backline there etc.) the normal/inverted-footswitch is, hopefully, a good solution.
:-)