I just recently set up the nextCloud calendar app.
I wanted to import an ICS file from my old calendar through the web interface. Unfortunately, it just says “351 imports failed”. There is no other error message, some events do seem to get through though, but none of those have anything special about them…
The ICS file is a consolidated file from about 400 single files. It’s correct ICS, validation in a validator succeeded. If I upload every single file it works just fine, but changing the calendar for 400 files… thrice… I’m sorry, but that’s not going to happen.
OK, I’m 4 years late but I’m sure someone still has this problem. I had the mistake to import from a mobile phone calendar. You can edit the ICS file manually, but it’s not much fun, I first imported the calendar into the calendar of Thunderbird and then exported it back as .ICS file. After that it could be imported into the nextcloud without any problems, including all memories. Certainly does not solve every problem, but it is worth a try!
Just wanted to thank you for this answer. I was breaking my head to find a solution for this, and this was super simple and easy to implement. I recommend this solution for people having trouble manually importing ics files to Nextcloud.
In the year 2021 exporting and importing in thunderbird did not help with Nextcloud 21.0.3 .
I tried it on chromium based brother with no success.
Only using firfox worked!(i found the tip here No calendar import · Issue #1898 · nextcloud/calendar · GitHub)
You need to open calendar in firefox and then import and wait a very long time. after like 1h the calender seems to be imported