Alright, here’s the whole story.
I want to add a trusted certificate to a “unifi controller system”. To do so, I followed the instructions on https://help.ubnt.com/hc/en-us/articles/212500127-UniFi-SSL-certificate-error-upon-opening-controller-page.
Instead of buying a ssl certificate, I decided to use Let’s Encrypt. But apparently, the .pem (and .der) files created by the ace.jar program cannot be parsed properly.
I can post the .der too, if it would be of any help.