Ok, an update:
In "Updates" under "Ubuntu Software" I see an update named "XPS 13 9370 System Update" from 0.1.5.1 to 0.1.6.3. Oddly, the description list a MS-Windows issue. Weird. I am not sure if this is the case also for factory-default setups. My main issue is still the one at the message above, however.
Here is the description of this update:
"This stable release fixes the following issues:
• Fixed the incorrect Windows operating system logon message that is displayed on the lock screen of Windows.
• Fixed the issue where the characters fail to display on the screen while typing continuously.
• Enabled support for Intel Turbo Boost Technology on I3-8310U processors that was disabled by default.
• Optimized the battery diagnostics in Dell Enhanced Pre-boot System Assessment (ePSA)."