Lightroom Desktop: Unable to export images

Images are synchronized from Lightroom CC on another PC.
If I use lightroom on my phone, it is no problem, export and synchronization work perfectly.
After the last update to 4.2 (Automatic) the problem started.
All files are local on the server and are synced to the cloud (with the mobile app on my phone everything works properly)
Lightroom 4.2 does not show images
Lightroom 4.1 shows images but export will not work.
Next action taken.
(Windows 10) Locally deleted all files, registry entries and images. (clean installation)
No images with 4.2
Again Locally deleted all files, registry entries and images. (clean install 4.1) Images are "downloaded" but still no export possible.

A Preference File will survive a Lightroom uninstall/reinstall. Sometimes weird behavior is corrected/cured by resetting the preferences.

Reset Procedure:

1. Close Lightroom.

2. Hold down [Alt/Opt]+[Shift] while restarting Lightroom.

3. Overwrite the Preferences when prompted by the dialog.

4. Close Lightroom.

5. Restart Lightroom.

Does the behavior continue after resetting the preferences?

This is not a solution. With version 4.1 I still can't export anything.
Version 4.2 cannot connect to the cloud, nor does it show any images.
In the versions now completely reinstalled started with shift / alt then closed PC restarted and lightroom started.
The images can be exported with LR mobile and Lightroom CC

What is your current OS Version Number?

Do you have a firewall whose settings may need to be updated to accommodate 4.2

Do you get any error messages you can share?

Windows 10 Pro N
Version 20H2
build 19042.867
Firewall disabled on PC makes no difference for 4.1 and 4.2 behavior and problem is the same.

no error messages on 4.1 but export starts and hangs.
With 4.1 I can sync images with the cloud (no export, no error messages). After this update to 4.2 even images are no longer visible. If I look at settings, I see the meldin not connected to the cloud

Time to get a diagnostic log then (from 4.2)

For LrDesktop

Go to Preferences

Account Section

Hold down the [Opt/Alt] key to make [Diagnostic Log] button appear.

When creating a log file in version 4.2, it "hangs" halfway. This must be done after half an hour.
Log file 4.1 has been created, which files am I going to upload and how?
Just for the sake of completeness, just reinstalled 4.0.
There are no problems with this version. Can export and see all images.
After an update to 4.1 I can see all the images but I cannot export them.
After an update to 4.2 I can see the number of images in the catalog, but I don't see any images.

I have emailed you to obtain the 4.1 diagnostic log. 

Make sure the log was made after a failed export. 

The file arrived. 

We only need the diagnostic log and it needs to be in Zip format.

Can you resend after the export error.

For LrDesktop

Go to Preferences

Account Section

Hold down the [Opt/Alt] key to make [Diagnostic Log] button appear.

The engineer reviewed the new files you sent. Thank you. 

Question: Have you done anything recently around Watermarking? If you clear watermarking in the Export settings, do the files export? 

This could have happened on another device. Any watermarking work at all?

Version 4.0 with watermark just export the photo.
Update to 4.1 via cloud, same settings as 4.0
version 4.1 without watermark export works, with watermark no export.
Watermarked image (* .png) redownloaded, still does not export the photo.
(LichtroomMobile works fine with this watermark.)

Can you forward to me the PNG file that you are using?

If you delete the watermark from Lightroom (4.1) allow those changes to sync, will 4.2 install, launch and show all the images?

if not,

if you create a different watermark with a different generic PNG file and allow it to sync will will 4.2 install, launch and show all the images?

Note: you may also need to remove the watermark from your other synced devices temporarily. 

Tried all of the above but still no images in 4.2

We have a second bug logged for the Watermark causing Export to fail. 

At the moment I am using 4.0 so this export does not cause a problem, not even with the watermark