I’m still experiencing the problem with Thunderbird (v45.8.0). I even tried with a completely fresh install of NextCloud (v12.0), with no difference: if I try to subscribe to a calendar using Lightning in TB, it is quickly disable and greyed out…
As suggested in this thread, I tried setting network.http.spdy.enabled.http2 to false, but it did not fix it. I am now wondering whether that could be related to OpenSSL (v1.0.1e-fips, which may not be enough).
Note also that on the same server, I also run an old instance of ownCloud (v8.2.9), which syncs the calendar without any issue (using HTTPS too).
Any feedback or idea greatly appreciated.