I certainly was figuring how to change the color of the calendar, but as far as I can recall, I could not find anywhere in the Preferences. I did switch to the System Theme from the Preferences.
Do you foresee a connection of this problem with the color of the calendar?
Hi it shouldn’t be connected to the theme changing, but if you ever right clicked a folder of your calendar, selected properties and clicked on a different color for your calendar that would probably be the cause of this issue.
Unfortunately it’s not supported by Outlook.com, we’re working on a fix for this issue.