Problem with linking to post

You can take out the “index.php” and you’ll get the same result.
The reason:
– the second URL (cat=5) doesn’t use the physical index.pgp file but its own category template
– seemingly you don’t have a single.php template file, so the single post view is trying to use your index.php file… which (I’d bet on this) doesn’t have a Loop.

No Loop > no posts displayed.

I’d suggest you do some reading:
https://codex.www.ads-software.com/Template_Hierarchy
https://codex.www.ads-software.com/Teme_Development

Ah great, I created a single.php file with a loop and now I can see the content of the specified posts in the url.