Error 403 on saving markdown page that contains the String /etc/

[sttzr]

I'm currently trying out Wikitten as a personal markdown-Wiki and like it very much.
It's just that I always got an Error 403 on some .md-Pages when I tried to save changes. Now I narrowed it down a bit and found out the following:

• It fails as soon as I type the String /etc/ somewhere in the document.
• It doesn't make any difference if it's inside a fenced code block using ``` or not.
• Markdown pages that I upload manually using ftp are rendered correctly even if they contain the /etc/-String.
• It's only when I click on "Toggle Source" and then clicking "Save Changes", that the error occurs. When removing just one character from the string it saves successfully.
• In my Developer-ToolsI can see a POST-request to /?a=edit beeing made that contains the markdown content failing with Error 403.
• Same thing in Firefox or Chromium.
• My server settings are php 7.4.6 served as FPM via Apache.

As far as I know it should be perfectly valid to use the string /etc/ in markdown syntax without further character escaping. So I assume that there is a bug in Wikitten.

Can anyone reproduce this issue?

1. Create a new page test.md, as content type blablabla and "Save Changes"
2. Now edit the page and add /etc/ to the end.
3. Do you get an Error 403?

Thanks a lot!