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

[kmueller2]

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.

[HelloZeroNet]

It happens when you have a javascript error. Check Preverse Log in chrome js console (F12) then see the error message when you clicking on the button

[kmueller2]

thanks for replying so fast.
The error I get is:

this.tag_markdown is undefined

I suspected that MathJax was to blame so I switched it from the header to the footer then removed it completely but the error persists.
What is funny is that when I create a new, empty blog and edit a post there this error does not appear. However I see no difference between before and after clicking this button.