Found a solution for Firefox
(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.
(2) In the search box above the list, type or paste ocsp and pause while the list is filtered
(3) Double-click the security.ssl.enable_ocsp_must_staple preference to switch the value from true to false
After this the https://rendezvous.creator.kodular.io/ should load and show
# Kodular Rendezvous Server
This is the Kodular Rendezvous Server, which appears to be operating normally.
Once this is confirmed, you should be able to use companion
Mark this as a solution if it works for you