"Forbidden. Wrapper nonce error" when switching to markdown in blog

Got this persistent issue today. I click on the "switch to markdown" button on the right side while editing a blog post and the above mentioned site and error pops up.
Full disclosure: I am currently trying (and failing) to get MathJax to interpret some formulas in the post. Installation is local, however because of the huge size of MathJax I had to set the size limit quite high, maybe this influences this error.