"PROPFIND is not allowed"

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That’s interesting, I get a 404 in my browser as well. In Thunderbird, Lightning accepts the link and syncs everything in the calendar.

Maybe with another port or something like that? Is CalDAV and its path documented by your provider?

No, they only provide the link, and a guide to add the calendar to Outlook/iOS/macOS, so that’s not helpful.
Does Lightning check other ports then 80/433? Maybe I can check the Thunderbird/Lightning debug to see what it is going on.

@Alexhasahorn Yes, that would be helpful. I have tried your URL with PROPFIND and it returns 405, so the error is not only on GET (which is used by browsers).

I’m a bit out of my depth here, maybe you can make sense of this:
This is the Stacktrace: https://pastebin.com/E1E1cZDr
This is the headers/response: https://pastebin.com/tbgLP3kS

Do you use the same Internet connection with Thunderbird and your mobile phone?

Nope, same WiFi network. Thunderbird works at home and at work.

Got it (thanks to the log with request/response headers)

$ curl -I https://webmail.mijndomein.nl/caldav/Y2FsOi8vMC8xNTY/
HTTP/1.1 404 Not Found

$ curl -I --user-agent 'Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.2.1 Lightning/68.2.1' https://webmail.mijndomein.nl/caldav/Y2FsOi8vMC8xNTY/
HTTP/1.1 401 Authorization Required!

So your server distinguishes clients by User-Agent and locks out browsers and DAVx⁵. However, it does not use a 403 response with a message like “Forbidden with your client”, but disguises the CalDAV service instead.

This is a server configuration problem or maybe even a “feature”. Can you contact your provider about that and tell me what they say?

Great! I guess this is a ‘feature’, but I contacted them, I’ll see what they say.
I’ll keep you posted!

I emailed this to support, and they referred it to their 3rd line support. I guess they will fix it - or not. There is not much I can do now but wait and see if it starts working at some point.

I guess this is not a common problem, otherwise some user-agent spoofing option could fix this more generally(?)