I'm working on a client website project. The site is quite simple:

Backend: TYPO3 v6 (running on PHP7) Frontend: jQuery v.1.9, jQuery FlexSlider v2.1

So basically the front page is one page and the navigation just scrolls to specific anchors on the site.

For a few days the following strange behavior happens when a visitor comes to the site:

The site finished loading and when a user clicks on some link on the page, the whole page is refreshing and the JavaScript is kind of broken. (And so the UI as well)

A flex slider gallery disappears (on Firefox) and there are fold-in/fold-out divs that do not work anymore.

The chrome dev console gives the following error:

Uncaught DOMException: Failed to execute 'pushState' on 'History': A history state object with URL 'https://example.deindex.php/?id=1#cB4' cannot be created in a document with origin 'https://example.de' and URL 'https://example.de/index.php?id=1#p4'.
at Function.m.pushState (https://example.de/typo3temp/compressor/merged-73956f754c7b34d423c38b286c1f15e4-77054febad955a7df4cd7cae818d47b1.js?1635547657:263:12820)
at HTMLAnchorElement.<anonymous> (https://example.de/typo3temp/compressor/merged-73956f754c7b34d423c38b286c1f15e4-77054febad955a7df4cd7cae818d47b1.js?1635547657:318:13)
at HTMLAnchorElement.dispatch (https://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js:2:5418)
at HTMLAnchorElement.y.handle (https://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js:2:2126)

In Firefox the error message is

Uncaught DOMException: The operation is insecure. merged-73956f754c7b34d423c38b286c1f15e4-77054febad955a7df4cd7cae818d47b1.js:263
pushState merged-73956f754c7b34d423c38b286c1f15e4-77054febad955a7df4cd7cae818d47b1.js:263
<anonym> merged-73956f754c7b34d423c38b286c1f15e4-77054febad955a7df4cd7cae818d47b1.js:350
jQuery 8
<anonym> merged-73956f754c7b34d423c38b286c1f15e4-77054febad955a7df4cd7cae818d47b1.js:347
jQuery 9
<anonym> merged-73956f754c7b34d423c38b286c1f15e4-77054febad955a7df4cd7cae818d47b1.js:282

I already cleared the whole cache of TYPO3 and even rollback to a backup where everything was working fine but the error is still present.

I think it can be related to the server configuration?
Like something wrong in the .htaccess configuration?

The page is served over HTTPS.

BaseURL configuration in TYPO3 is

[baseURL] = https://example.de/ # RealURL

I'm very thankful for every kind of help!

P.S.: I know the page tech stack is totally outdated, I already highly suggested updating everything or better relaunch the page because everything is like from 2012/2013. But right now they just want to get the page working again.