Are you talking about scheduling "one time" downtime or recurring downtime?
One-time
Were you logged in as nagiosadmin or another user?
Multiple users reported the issue to me. I also checked and confirmed the issue.
What kind of user (security settings, local or AD/LDAP user, etc.)?
All AD users
What is the date format that is being used (user's & system-wide settings)?
yyyy-mm-dd hh:mm:ss
Did you try restarting nagios prior to rebooting the server?
I did an apply configuration between the first report and last test before reboot.
Can you schedule downtime successfully now or you are still having issues?
Yes I can schedule downtime again.
Did you see any error messages?
Had no time to check.
It would be nice to at least know immediately when this happens. Maybe some sort of check which puts a dummy host in downtime, waits 5 seconds and then checks if the host is in downtime?