Okay, I can reproduce the issue when accessing your server and I think I know what goes wrong.
When accessing /index.php?module=Proxy&action=getCoreJs Firefox just gives a general “Content-Encoding-Error” without more helpful information. Chromium shows net::ERR_CONTENT_DECODING_FAILED which brought me to
Weirdly enough curl https://piwik.yourmatomo.example/index.php?module=Proxy&action=getCoreJs works perfectly fine. But this brought me to the solution:
When a client requests that the server sends the respond compressed (which all browsers do with a Accept-Encoding: gzip, deflate, br header), your server responds that it compressed the response with gzip (Content-Encoding: gzip) but the actual data it sends is not actually compressed but still plain text.
You can reproduce this with curl -v https://piwik.yourmatomo.example/index.php?module=Proxy&action=getCoreJs -H 'Accept-Encoding: gzip, deflate, br' and you will see both the Content-Encoding header is set and the response is still plain text.
Or you do a curl https://piwik.yourmatomo.example/index.php?module=Proxy&action=getCoreJs --compressed where curl fails to decompress the stream as it is not actually compressed.
* Error while processing content unencoding: incorrect header check
* Failed reading the chunked-encoded stream
As to why your webserver would do this, I can’t give a clear answer, but I would check your Apache config exactly (especially the parts related to gzip and compression). Maybe something is set up strangly there.