I faced with this issue for calendar. I had my calendar synced with Nextcloud. In one of the calendars, I added an event on my phone (which uses Persian language) and set reminder. Thereafter, I had sync issue.
Today I looked at the .ics file and saw this:
You can clearly see on TRIGGER line, there are Persian numbers.
I manually edited the file and made a new calendar on Nextcloud. It is working fine now.
Just the point is title or description shouldn’t be modified. Only those lines should be modified which use number for some calculation.