"PROPFIND is not allowed"

[Alexhasahorn]

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

[rfc2822]

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

[Alexhasahorn]

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.

[rfc2822]

@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).

[Alexhasahorn]

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

[rfc2822]

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

[Alexhasahorn]

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

[rfc2822]

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?

[Alexhasahorn]

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

[Alexhasahorn]

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(?)