After updating to the new version of the Mobirise Website Designer, the GUI looked ugly. There was obviously a wrong font, making all GUI–labels and other displayed text unreadable.
Luckily, the default footer of the demo website uses the same font. Checking with the developer tool in Chrome browser, I found its name: „Raleway“. Conveniently, it the „League of Movable Type“ published under a free license, so downloading and installing was easy.
Font available on the system, problem fixed.