# Open Thunderbird: Tools > Options, select the Advanced panel, select the General tab, and click Config Editor…
# after open Config Editor… (about:config)
# search for „network.cookie.same-site.enabled“
# change: network.cookie.same-site.enabled = true to -> false;
# Restart Thunderbird and retry connect to server
https://github.com/nextcloud/server/issues/9869