Adding nw calender to the IMac calender creates error message

On my IMac I want to add another calender. The one I also use on my Iphone. I get the following error message:

Het verzoek voor account 'iCloud' is mislukt.

De server reageerde met

'400'

op de bewerking 'CalDAVUpdateShareesQueuableOperation'.

Strange also is the fact that the calender already has items from the calender I want to include. The items that are given are only the recurring items.

Please help