It’s clear now. It’s a BBpress issue.
When I disable the BBpress plugin, logging in is no problem anymore for the imported users.
So BBpress 2.6 RC3 is not compatible with WP 4.8.2.
In addition to the my question above:
It seems that I can log in with my mailadress instead of my username. I saw that the BBpress plugin has not been tested with WP version 4.8.2. Could this be the issue here?
Thank you very much for your reply.
So most likely we will stick to 2.6 RC3 for the time being, and move to Final release when it is released.
Thank you again for your advice.
It looks like the file is not at that location anymore. Any expectations around the final 2.6 release?
Where can I download 2.6 RC5?