I just installed a Raspberry Pi with the latest version of openHABian & openHAB 2.4.0. together with AVM FRITZ!Box Binding and connected my Fritz!Box successfully.
In the thing configuration of the Fritz!Box I chose the protocol HTTP to establish the connection between openHAB and Fritz!Box. I would like to change it to use HTTPS instead. But when I change the connection from HTTP to HTTPS it does not work any longer. This is a screenshot of the log:
Yes, the reason is the certificate. I am afraid you cannot use HTTPS locally. Only in combination with MyFRITZ! Service for remote access. Currently there is no “trust all” button in the framework. It would be possible to implement an own TrustManager for the binding but each FRITZ!Box provides its own certificate - either self-signed or from Let’s encrypt - the openHAB user would have to upload it somewhere. That is a little bit tricky.