It was working fine until yesterday, but today, when you add a new topic, it doesn't redirect to the forum page, but to a blank page. Any ideas?

Just Restart your server and try again. I was having same problem.

It only could mean your server gives back some error, but since errors are suppressed in PHP settings, you don't see the error itself, but the blank page. Try to locate error logs of your server, there should be some point to the error line. There may be tens of reasons of why it's not working, but it's definitely your custom issue, not a miniBB bug.