Sometimes the website is not displayed correctly

Dear Drupal Community,

it happens occasionally that our website ( is not displayed correctly.
So pictures are missing and the central positioning of the content is corrupted, so there is no padding on the sides. This can happen all at once (picture missing + positioning of content messed up) or separately (either pictures missing or positioning messed up).

I have a screenshot as an example:

Now we don"t know what causes these issues, sometimes the website is functioning perfectly fine. The errors occur no matter on what kind of device or with what kind of browser the website is accessed with.
So if the errors occur, they occur for everyone for the same period of time. So the problem must be on the website/server side.

Now what we discovered is, while using the navigation of the site to get to another sub page, the URL somehow changes. So instead of accessing the URL automatically changes to (see screenshots). We have no clue what causes this insertion.

We suspect our theme, the Porto theme (I know…), might have something to do with it.

Have any of you encountered a similar problem and maybe even found a fix?
Thanks in advance!