Import with error: " exception"

When i try to import 14 Calenders from BusyCal-Export to an NC 13.0.2, 1 of these Calenders can not be imported with the Message “ exception”. The other 13 Calendars are working fine.

I searched for this Term and found it on my Harddisk in the NextCloud-Manual.pdf, so i searched for this here at the Forum. But not with my Problem.

So i ask here: whats wrong and what can i do, this one Calendar?

Server is at al-inkl, Apache, NC 13.0.2, PHP 7.1

P.S.: please sorry for my cruel English