Thanks for quick response.
The prod version is 19.1 without docker, only contain source from github. The webserver is Apache2 with configuration of 17.1, but i was tested for many configuration option in vhost. The most of php version is 7.3 with necessary moudules, but i tried from 7.1-7.3.
I have 4 different installs:
Docker 19.1(original apt install without modification)
Test environment 19.1 without docker (Apache+mysql+php7.3(tested 7.1-7.3))
Prod 19.1 without docker(apache+mysql+php7.3)
The problem occurs after database migration (with /setup backup-restore) from 17.1 to fully fresh install 18.04.
Thunderbird + iOS clients cant response to the invitees if the caldav account is newly added and iOS client calendar inbox is empty; old clients that was configured with 17.1 can access without problem.(i guess this from cache)
All new 19.1 (docker, without docker) installs contain this problem independently php version or apache configuration.
Installed prod php moudules:
Otherwise the new Egroupware is really nice and good performance in my testing:)