Problem solved. I didn't find a place in Github/M5Stack where I eventually could create an issue. However, we are here in UiFlow 2.0.

Thanks for your assistance robski!
Paulskpt

Additional info:
First, in the online Uiflow2.0 page I deleted the existing registration of the M5PaperS3, so M5Burner would go to ask me to bind this device to my account.
I reflashed UIFlow2 using M5Burner V3 (beta-win64) (I googled for an older version however I didn't find). During the burn process inside M5Burner I was asked to bind the deviced. I did and accepted the default option "Public". After a successful flashing I disconnected the M5PaperS3 from the desktop PC. I rebooted the M5PaperS3. I saw on the display of the M5PaperS3, main window, under "MODE": "Private" and under "ACCOUNT": "None".
I remember from using other M5Stack devices in Uiflow2, that it can take some seconds, after the device has connected to the M5stack server, to update its account information on the Uiflow2.0 webpage and on the device itself.
However, this time with this M5PaperS3 I saw no account info update. On the desktop PC I started a Uiflow2 session within a Chrome browser. When I clicked on the device info, the Uiflow webpage showed MAC-address, a TOKEN value, a device-ID and Permissions: "Public". On the M5PaperS3, main window, I saw that the info did not change. It showed the same data as I wrote above.
I created a short video of the boot process. In this video one can see that on the bottom line, left side, two icons appear. Left: a WiFi-icon. Right next to it, a Server-icon. Initially the Server-icon looks like what I interprete as "connected". A few seconds later this Server-icon changes. From then on the Server-icon has a "x" in it. This I interpreted as "disconnected from Server".
In the Uiflow2.0 webpage, the icon of the M5PaperS3 device is shown with a red dot and a red question mark ("?") inside of it. When I clicked on that logo, there appeared on the right-top of the UIflow2.0 window a pop-up message window saying "Error: the device type is unknown".
I don't think there is a WiFi problem with this M5PaperS3. Why? Because, after I had built and flashed uiflow-micopython firmware to this M5PaperS3, I created a MPY sketch that connected to a WiFi Access Point here in the house and have the device connecting to a NTP-server and ask for an NTP-datetimestamp which was received OK. Conclusion: the WiFi of this M5PaperS3 is working.
Sofar my response. I don't know what I can do more. Maybe create an issue on Github.
FYI here is a link to the video.
Regards,
Paulskpt