Steps to reproduce:

1. Configured CalendarServer-9.0(https://github.com/apple/ccs-calendarserver/releases/tag/CalendarServer-9.0, https://www.calendarserver.org)

2. Added 2 users, postmaster@test.int and abuse@test.int

3. Configured above users account in two separate Thunderbird + Lightning client

4. Created meeting invite in abuse@test.int's account and added postmaster@test.int as attendee

Actual results:

1. Meeting is created in abuse@test.int's account

2. Meeting is synchronised in postmaster@test.int's account

3. postmaster@test.int is not able to accept meeting invitation as no accept/decline buttons are shown

Expected results:

1. postmaster@test.int is should able to accept meeting invitation

Side Note: 

1. With same setup, configured iOS clients(iPhone and iPad) and Android clients(CalDavSync)

2. Confirmed expected result