Calendar disappeared

The checkbox for my CardDAV calendar is disappeared. Synchronisation shows no error message an contacts on the same account is working fine.
I already deleted the account and reinstalled it. No change. Contact synchronisation works but no calendar checkbox.
Any ideas?
Best regards, Michael

CardDAV is for contacts not calendar.

Which checkbox disappeared? Is this in the account settings, or in the calendar tab?

In the Calendar tab.

Could you maybe give a screenshot?

Who is your CalDAV provider?

My provider is posteo.de. I use the german localization of em-client. Screenshots are below.

OK. The check boxes are there - red, orange, green, blue, yellow and purple. Just click on the box and the tick will appear.

Sorry. The tick did not appear. The coloured boxes belong to my Apple-Account (xxx@me.com). I need the checkbox for my account at posteo.de

Did you setup a CalDAV account? If not go to Menu > Tools > Accounts and add a new account. Choose Calendar > CalDAV then enter the url and login credentials.

I did a reset of all my accounts on another PC and did a backup. After restoring the backup on my PC everything worked fine.
Thanks for trying to help me.