Hey guys,
I solved my problems. It has nothing to do with the Shelly Timer itself, which is working without any problems (so there are no low limits at 256 or whatsoever). I also tried your idea ostfriese and had the same problems as before until I found my error.
My function to be looped by the timer contains a http.get command embedded in a Shelly.Call command which in some cases didn't work reliable. After some time, there were no responses to these requests but the timer continued anyway. As the max number of parallel instances of Shelly.Call is limited to 5, the script was force stopped. It took some time to find this error.
However, thanks for your support
Best regards
Sven