==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 0.9.8
- Linux/BSD distribution name and version: Ubuntu 16 LTS
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): Mysql
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? No - But thinking we will  - easier.. :-)
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

Mac mail and mac calendar.
IMAP usage on mail, and caldav using sogo.

We get an error back from the calendar app with a dialogue window saying:

The server responded with
“500”
to operation CalDAVWriteEntityQueueableOperation.

Also, the person who sent the invite receives approximately 9 emails saying the invite has been accepted....

Any thoughts?

thank you