I have the same problem. But "new topic" is working after reloading the page.
Same thing with the correct theme display of forum topics.
Update. After clean cache all works fine, sorry
I noticed a little "bug" but annoying while browsing my forum on my mobile. I wanted to create a topic but I couldn't submit due to the upload thumbnail icon which overlaps the submit button. I've got a nexus 5 running android 4.4.4
Are you running the latest version? Try disabling some plugins in case they interfere.
I'm running 0.5.3 stable. Ok I will try tonight when I'll be at home.
Edit: @julian I tried to disable plugins, it's still doing the same thing unfortunately. The quick solution that I've found for the moment is to deactivate the upload of a thumbnail for topics.