Calender Group not working with Exchange 2013 SP1, Undefined index: SUMMARY

When I try to create a calendar group that refers to an on premise Exchange 2016 SP1 calendar, I get the error message shown on the first picture.
I'm trying to use the category function but no categories are suggested.
This problem has only occurred for about 3-4 weeks. Before that the function worked without problems.

With an Office 365 calendar it works without problems.