Today there was a Zwift update and so I installed it. After that went to a group trip. After the start, I noticed that the power, cadence and heart rate were displayed correctly on the screen, but the kilometers stopped at zero even though I was moving with the group. Also some other riders had different mileage progress. After about 20 minutes I finished the ride. Nothing was saved. I shut down the PC and restarted Windows. After that, I started Zwift and joined a group workout. Everything was fine again.
Important finding: After Zwift update, the computer must be shut down, otherwise it will not work properly for Zwift!