From a worldwide perspective, it is incredibly uncommon, and just from the basics of this issue it is clearly uncommon in Switzerland as well. The standard keyboard layout in Switzerland is QWERTZ (across all of its language speakers). This website is generally only used by the Swiss.

I mean, if a basic authentication mechanism of a critical government website doesn't work with a keyboard layout, odds are incredibly likely that said keyboard layout is uncommon. Like not one of the developers, testers, stakeholders, or even other users had a problem -- and this isn't a problem you can just ignore -- so it probably isn't that big of a deal.