TYPO3 internal links inserted from the ckeditor are not "converted"

TYPO3 internal links inserted from the backend are not "converted", they stay in the same format, for example an internal link will appear like this in the frontend: <a href="t3://page?uid=2">internal link test</a> I'm a beginner using TYPO3 8.7.8, and a custom ckeditor configuration. I have also extended the ckeditor with my own plugin. How can I find what is causing this?

Edit 1:
Seems like none of the links prefixed with "t3://" are working. Is it possibble that the LinkService is not called?

Solution

After uninstalling every non-vital extension and clearing every possible cache and a few hours of debugging we have found the problem: the "frontend_editing" extension - only if the user is logged in into the backend!