Thats an internal url so it should be stored like that in editor. Is the same link displayed to end user? Episerver should normally transform those to external links when rendering field to user

Yes, in some cases it displays the internal link to end user. We have an editor-block and some of them contains the faulty urls. I then have to delete the block and create a new one. I can copy the content, and even the link from the old and then it works. Can it have happened when we upgraded the cms from 9 to 9.4?

Checked sitesettings in admin and the domain name there?

Done any custom routing?

Yes sitesettings are ok. No custom routing done!

Exported content from another site which breaks internal links if not all targets are exported as well? Like a staging environment or similar... 

No, we have not. It shows upp randomly on editors and editor-blocks. To get rid of it we have to throw away the page or the block an recreate it. In some cases that does not work either.

Did you switch to https at the same time?

We switch to https on our new site, yes. But we have that configured in sitesettings in admin and the domain name there. And this problem does occur when we create a block and links internally within our new webb on the new server.

Got anything special in tinymce editor that might mess up parsing the html? Or does it occur even if you are just using simple paragraphs?

Checked logs for errors?

Can you try feeding the internal url directly to the UrlResolver and see if it returns a friendly url in code? If it does, you know routing works, then it's a parsing problem most likely or vice versa.