Same issue as here: Unable to create event
You should REALLY change your global LC_CTYPE to an UTF8-based locale (seriously, why are there still systems in the 2020s that do NOT use UTF8 by default?!), but setting the env var for both indico-celery.service and indico-uwsgi.service should do the job:
Environment=LC_CTYPE=en_US.UTF-8