I'm not sure when it stardted, but I suspect it has something to do with the installation of a plugin:
when I install a new plugin (say: calibre), the associated web interface would be http://server:1234 (1234 a non-standard port). The problem is, that all access to this URL is instantly redirected to https://server:1234 - a port nothing is listening on.
what could be the reason here?