Merijn Schering
Merijn Schering
Sounds like an outlook problem but it might be worth checking the server logs to see if there are any errors server side: https://groupoffice.readthedocs.io/en/latest/troubleshooting/logging.html
I can't replicate it. If I set a date with 0:00 it's saved correctly: 
Thanks for reporting. It has been fixed for the next release. I will create one today.
Yes we are aware you can only add contacts. A new calendar is in the works where you can add any e-mail.
Will the Sender header make a difference here? Won't SPF fail anyway because it will check the return path? https://mxtoolbox.com/dmarc/spf/setup/spf-return-path
This one was fixed some time ago
It's the manager that is set at the users account in the "Employee" tab.
You say you get it from time to time. What are you doing at that time? Could you enable debug and send us a stack trace?
Good news! This will be implemented in 6.9.
Are you using PHP 8.3? It's a compatibility issue. I've fixed it in the attached commit.