Calendar - How far in advance greyed out

Hallo,I have an user, who has problems with the alarm-function of his calendar. If I enter a reminder for today, alarm works. But anniversaries (which have been maybe entered in an older calendar version) don’t show up, also “Notify Me” is set. One strange thing I found is, that under Tools - Preferences - Alarms, the fields “how far in advance” are greyed out, and I can not change anything here. Any ideas what that could be? Thank you.

Oliver