It took me a while to figure out the solution.
I found the final hint in TMS Alert viewer, since the solution manager is a DEV system with a lot of history there were a fair bit of unrelated alerts. We cleaned up all errors until one remained:
RFC system error in system/destination for user DDIC in client 000.
We first analysed the RFC connections used, especially TMSDPL@<SID>.<DOMAIN> without result, when I recalled that there was mention of DDIC in the cCTS How To Guide "How To... Set Up cCTS for ChaRM and QGM".
When resetting the RFC connections according to the guide the problem resolved itself.
Always remember: RTFM ...
I saw that the thread is getting some attention, so I posted the solution.