@debbe you can get a Lite-Cloud-License for free if you complete the ETS6 Web Training:
https://wbt6.knx.org/login/index.php
Just register with fake name/address/phone etc., complete the course, fill out the survey and you’ll get a cloud license which can sign a project with up to 10 devices. There should be a possibility to trick the ets6 to use that lite license to sign (or certify, as thats how its called in the ETS6 code) a project with more devices while exporting.

I think the issue is that the converter from ETS5 to ETS6 also checks for a valid certificate. That could possibly be patched out in the “Knx.Ets.CvNext.dll” File.

Importing a cracked ETS5 .knxproj with 20 devices does work for me (ETS6N), so its perhaps a ETS6C issue?