[FAIL] Receiver: down ... failed!

This is not a problem with FR24 though but rather looks like a dump1090 problem.

My graphs might help both of you check that your local dump1090 continues working:

If it's indeed dump1090 that stops working, i'd recommend switching to dump1090-fa:

(you'd need to reinstall the graphs when switching to dump1090-fa after installing the graphs)

With a separate dump1090, problems can be more easily separated beteween fr24feed and dump1090.
Also there are dump1090-fa logs which one can check.

I'm not sure how fr24feed behaves when the dongle has insufficient voltage.
You should check if your RPi has sufficient power with this command:

dmesg --ctime | grep voltage

If there are under-voltage messages this points to an insufficient power supply which can lead to all sorts of problems with dongles.