I have filed now a bug at the jstz library, because I guess that would be the cleanest solution.
If that ignores Etc timezones coming from the internationalization API from the browser, then it calculates a proper Olson timezone.
it does not depend on Windows Server version.
I got more details: the client is Firefox 56 running on Windows Server 2012 R2. The timezone setting for the user in Roundcube is "auto".