Import of vCalendar File from OwnCloud works partially only

I want to migrate from ownCloud (9.0.8) to Nextcloud (11.0.3).
So I installed Nextcloud to a different directory and exported my calender entries from ownCloud to a calendar file.

When I import the file to Nextcloud then import starts but at the end I get the message “Partially imported, 78 failures”.
Tried the import a few times and also with different browsers (Firefox and Chromium) and everytime the number of failures differs from one attempt to the next. Before every attempt I deleted the calendar and created a new one to not confuse the import process with existing/duplicate entries.

Because it’s always a different number of failures I assume that there’s no invalid data in the calendar file but more a problem in the import process itself.

I dug a little bit deeper:
had the console of my browser open while testing a new import.

Import routine seems to create a HTTP PUT request for every calendar entry in the file and sends all the requests at once (or at least a lot of requests in parallel).
This causes the webserver of my webhoster to reply with “503 - Service unavailable” for some of the requests.

See attached screenshot of firefox web console output:

Is there a possibility to reduce the number of PUT requests that are sent in parallel to the webserver?

Contacted support of my webhoster and opened a ticket.
Information from support was, that my hoster has a limit of 8 parallel processes that are allowed to run in parallel per customer in their shared hosting environment.

So for a vCalender file that contains more than 8 entries it’s likely to reach this limit during import. And the more entries are in the calendar file the more likely the processes limit gets reached.

So it would be fine if I could limit the number of requests that are made in parallel.

I have the same issue with my web host. Calendars not fully importing and the same with address books.