Hello Oscar,
It took some time but in the end we found what was wrong with the dropdown and/or responsive design.
The CERN theme inherits the bootstrap theme. In order to better control the bootstrap version, we have locked the version so that it won’t get the latest one. The actual issue is that the bootstrap version was loading twice, once from the theme and once from CDN.
As you can see in the attachment above there is an option on bootstrap theme settings that enables the CDN provider that gives the latest version. In our websites, we have it set to “-None-” so that it won’t get the latest bootstrap version.
The websites that the dropdown was not working had this set to a CDN provider and as a result it was getting the latest bootstrap version.
Kostas
Credits: @pgroeger for spotting the fact that bootstrap is loading twice.