Hello @vilinski and welcome to the SonarSource community!
The SonarQube binding functionality does not support cases where authentication credentials have to be sent into Haproxy (or other reverse proxy) in order to reach SonarQube. We may be considering to add support for those cases if we see enough community users affected by this limitation.
To this extent, we will monitor if there additional comments or votes to this community thread (I renamed the title for more clarity).
Authentication to SonarQube from SonarLint via username/password is deprecated but, as you found out, it is still available for now in JetBrains IDEs. In order to investigate the null pointer exception you reported, I invite you to open a separate thread in the dedicated community section, with the SonarLint logs (here some info about how to get the logs).