Thanks for your feedback. If I call the backend.php (with the respective parameters) in my browser, the feeds are refreshed.
I checked some free cron services: All of them are stating, that they will quit a job after 30 seconds.
They recommend to add the following lines to the PHP script, to continue the execution when the job stopped:set_time_limit(0);
Will this also work for TT-RSS (if I add them to backend.php)?
If so, would it make sense to include this in one of the next releases?