Dear Noel
Thanks for your help!
Generating or regenerating doesn't help for RT2143 in my song. I tried a complete new song (with just a C chord). There it worked fine. So my earlier song has some problems with allowing solo RTs.

Then I pasted my chord progression in the new song, all tracks regenerate, but the 2143 is silent again after the first half bar...
I generated the chord sequence from a midi file; would that be an indication? Furthermore, I do notice (after pasting my chord progression) a red horizontal bar is present in the "1a" box on the top left. Another indication? If I check F5 (bar setting) nothing special is indicated on silencing the Soloist.