Thanks Paul. Actually I have tried to read the installation instructions carefully but somehow I must have overlooked the part about the slash. It turned out that the post topic button didn't work because I had my permissions set wrong.
After I have looked for the needed file permissions in the manual without luck I tried the miniBB support forum and found a topic
where you suggest that setting all permissions to 744 should be fine if you are not using add-ons. This is how I have set all my files and folders but appart from the "Post topic" button not working I noticed that images were not displayed either. So I set my folders (folders only, not the files) to 755 and now everything seems to work correctly. IMO it would be a very helpful thing to mention the needed file permissions in the manual.
I have also described my problem and solution in the mentioned and linked thread.