I would have set LANG=nl_BE.utf8 only in the shell before executing upgrade.py or only for this one command e.g.
But at the moment this is irrelevant.
I'm nor very versed in the inner working of OctoPrint or Python but if systemd doesn't find the executable the installation is borked.
Oct 01 18:33:35 raspberrypi systemd: octoprint.service: Failed at step EXEC spawning /home/pi/OctoPrint/venv/bin/oOct 01 18:33:35 raspberrypi systemd: octoprint.service: Main process exited, code=exited, status=203/EXEC
In the middle of this there is something missing, a copy&paste error:
/home/pi/OctoPrint/venv/bin/oOct 01 18:33:35
Before "Oct" there has to be a line break so at the end of "/home/pi/OctoPrint/venv/bin/o" is something missing.
If you can restore a working OctoPrint from the bak dir I would recommend it. Maybe a new try with the current version of upgrade.py as @Charlie_Powell suggested.