It's version 60.3.0esr (32-Bit) which should be the latest. OS is Windows 7.
The console says:
Error: fetch 'https://beluga-online.de/181203/data.json' response returned 401 Unauthorized c3runtime.js:2:97606
The problem appears just in Firefox ESR. Normal Firefox and all other browsers work fine. Also, it works with c2runtime without problems.
Maybe has to do with the htaccess protection of the page? If you want to look by yourself I can send you the password.