I have in office all users working with Thunderbird Ligtning callendar synchronised with Synology CalDAV server. CalDAV is configured on adres https://x.x.x.x:5006/ where x.x.x.x is public IP adress. By now all worked fine with Thunderbird 52.4. After upgrade to 52.6 calendar clients are not synchronising - login windows pop up many times and calendar has yellow triangle. If I change connection adress to local network adres https://y.y.y.y:5006/ of this same CalDAv server it is works fine (but onlu in LAN. I have try to upgrade to 52.7 and Ligtening 5.4.7 - still not work on public IP. Interesting is, that for new computer afer installation new Thunderbird 52.7 and Ligtening 5.4.7 everything is OK.