As this thread is the first hit in google and the problem seemingly still exists here is how I got around it:
I had the same issue while importing an ICS to nextcloud 16. The file was downloaded from NC 14. Each time I imported the file I had these messages:
104 of 1058 imported, 847 Errors, 107 duplicates skipped
108 of 1058 imported, 814 Errors, 136 duplicates skipped
88 of 1058 imported, 832 Errors, 138 duplicates skipped
77 of 1058 imported, 852 Errors, 129 duplicates skipped
The client (Browser) is simply to fast for your NC server. I looked up the network traffic and noticed a ton of 500 return codes.
In the developer tools (F12 → network) I limited the bandwith to “good 2G”, witch at least Mozilla Firefox offers.
With this setting I was able to import all calendar entries in one take.