This is the error in Nextcloud:

https://mail.....net/SOGo/dav/public/...@.../Calendar/personal/` resulted in a `501 Request Failed` response: <?xml version="1.0" encoding="utf-8"?> <D:error xmlns:D="DAV">no WebDAV GET support?!</D:error>

BTW this is a SoGo instance running in the Mailcow package

Have you defined domain-based users sources?

Sogo authenticates via Mailcow users databases - not sure what you mean with domain-based users sources. I am using the standard mailcow configuration.

Check if you have a domains dictionary defined in /etc/sogo/sogo.conf.

The only entries (which are not active) related to "domain" are these here in sogo.conf. Do these have to be configured manually to match my (multiple) Mailcow domains?

// Multi-domain setup
// Domains are isolated, you can define visibility options here.
// Example:

// SOGoDomainsVisibility = (
//  (domain1.tld, domain5.tld),
//  (domain3.tld, domain2.tld)
// );

You should perform a REPORT instead of a GET on a collection.

You must use the appropriate URL depending on your client. Use the .ics link if you want to subscribe to the public calendar.

I am simply adding this link "https://mail.mydomain.net/SOGo/dav/public/user@mydomain.net/Calendar/personal.ics to my Apple iCal client - it will throw an error. If I put it in a browser window and download, no problem - the ics can be imported correctly.

If I add the link above to Nextcloud Calendar, it will throw the above error. Can Sogo not support GET command instead of REPORT to provide the ics file? Not sure, but this way the interoperability is rather limited, if ics files don't work in standard applications.

So far it is the first time I encounter an issue that iCal could not work with an ics file, so seems to be incompatibility on SoGo's side to me.

The .ics file is .. a file, not a collection. From the Nextcloud error log above, you tried to subscribe to a collection instead of the ics file.

I can't replicate the issue with Calendar.app (File -> New Calendar Subscription...). Did you give public access to your calendar?

Yes, otherwise the download of the ics file would not work I think. Checked again, and indeed the error above was due to choosing the wrong link. However using the ics link in Nextcloud Calendar leads to this error. It is a 2MB sized file if I download it. The Calendar.app error does not tell any details when importing this link, it fails with a message after quite some time. Could it be Sogo is just too slow to deliver data with such a big calendar to share? It is strange that also the download of the ics file in a browser has quite some delay.

GuzzleHttp\Exception\ConnectException: cURL error 28: Operation timed out after 30001 milliseconds with 0 bytes received (see https://curl.haxx.se/libcurl/c/libcurl-errors.html) for https://mail.mydomain.net/SOGo/dav/public/user@mydomain.net/Calendar/personal.ics

@francis I can share the real link to you if you want to check the issue. Let me know where to send it to.

Any error in your Web server or SOGo logs?

I checked the logs now, but can't find anything in the SoGo log upon subscription of the calendar from Nextcloud.
I only see that there is a GuzzleHTTP client timeout in Nextcloud.... So may be an issue on Nextcloud side?

After further investigation it seems to be an issue that only occurs if calendars are very big. Eventually ics subscription should be limited to "last year and future events"...
Anyway, this issue can be closed.