Catalog doesn't import from another catalog

“Lightroom can’t import this catalog because of an unexpected error” 

I shot a festival last weekend, did some on site editing on my laptop. Now i want to import the festival catalog to my main one. 
specs of the main one and the festival match, they’re both:

Lightroom Classic-versie: 15.5 [ 202607291506-b8869fa7 ]

Licentie: Creative Cloud

I can open both catalog on my computer i can optimize them, close one, open the other all without problem. So they both seem to be working. I’ve put them in the same folder. So in folder x is a folder current catalog and a folder festival catalog. But everytime i click File → import from a different catalog, i select the festival catalog and immediately i get the error “Lightroom can’t import this catalog because of an unexpected error”. 

Somebody who can advise me?