Lukas,
Thank you for your comment on the users database we will keep that in mind for future reference.
HOWEVER, it turned out that the original problem regarding username case sensitivity/insensitivity is caused by a setting in the Config/config.yaml file!
We just needed to set "usernameCaseSensitive: true" and that solved our problem without reinstalling buru.
I updated the title of this question to better relate to the solution we were looking for.
Best regards!