Unable to reach the ACP, get redirected to homepage instead

Hi all. I know this shouldn't be a way to obtain support, but I've really been banging my head for days and I'm suspecting that there's something broken in the code base.

I installed phpBB 3.2.0 on my board, but I'm not able to reach the ACP. I can click on the ACP link and I can authenticate there, but then I get redirected to the home page instead of the ACP. I disabled all the installed extension, and the issue is present also in the original prosilver theme.

Is there something I can do to debug this?