@optimizingmatters Many thanks, mate! Just excluding jquery made other problems with the styling of the topic. I deactivated the Async JavaScript Plugin completely now and it works again.
Seriously the theme I am using – Shoptimizer – is a nightmare for an SEO like me. It is easy to edit for noobs, but it is a code monster. I guess I am “overoptimizing” it to have somewhat of an acceptable page speed.
Anyway, problem solved, thanks a lot!