Try a complete new install, download from top right next to your name pulldown from the download section latest version, install that one and chack again.

some quesses / tips:

Can there be interference from firewall? virusscanner?

Try to work localy, don't use external drives for x5 and projects, keep these on your default drive that is build in on your pc.

Andre, thanks for the advice, I did everything as you wrote, uninstalled and reinstalled the program, I have the structure for storing data and projects in the same location as on the original PC. But it's strange that peogram doesn't even allow to create new projects. It loads a new template, but you can't make modifications inside the templates. The only thing that works is the preview of the project I created earlier.

Can you test on another pc? 
Do you have admin rights on the pc that does not work proparly?

Are there any other programs that are active that might interfere?

Use win 11? or better what software version / operating software is running?

Did it work in the past?

Sorry for the many questions, just trying to find an opening for your problem.

If there was something with x5, the helpdesk would be filled with posts, so there must be something with the pc. That is my best best guess to get you hopefully in the correct direction.

Jiří, as Andre said, have you temporarily disabled or preferrably removed any 3rd party anti virus/malware/vpn or firewall? Also there has been a recent rash of problems with onedrive (and any other synchronising/backup software). Can you ensure that your working directory is not in onedrive as this can lock file generation (ensure that you are not working inside the onedrive folder c:\users\username\onedrive\incomedia or c:\users\username\onedrive\documents\incomedia)

Dear all, thank you for all your advice, but none has solved the page generation problems. The only thing that satisfied me at least a little is that I installed the program on my laptop and there, fortunately, it works as it should. I've switched everything that I could. That didn't help either. I'm using Win 10 as on the previous PC on which everything worked as well. I don't have Onedrive turned on and I don't use it. So don't know what to do.

Jiří, can you export the existing project? Go to step 5, export project (do not include backups or preview) and create an iwzip. If you can create it, re-open WX5 and select import and select the iwzip. This will NOT overwrite the existing project.

Assuming this problem is a recent occurance, do you have any older versions of  WX5 installed? If not, go to your profile (top right this page) and download (eg 2021.4), you will find your license key there as well. Install and try create a new project, does that behave properly?

It still sounds like something has taken a dislike to WX5, if you have removed all antivirus, malware, vpn, firewall protection, you may wish to run malwarebytes (or superantispyware, etc) across your PC just to see if there is anything running you did not expect.

Dear all, I tried to use all your advice, but unfortunately nothing led to a solution to the problem.

Did you install and use an older version, did it work?

When did / how long ago did WX5 stop working?

Are you using onedrive or 3rd party protection at all?

I have installed the older version but it is no use, the projects cannot be used, it says they were created by the newer version.
It stopped working a week ago, after installing on a new PC. But it works on the Notebook without any problems.
I don't use OneDrive. I use Kaspersky Anti-Virus 2020 CZ for antivirus protection.

Jiří, please either temporarily uninstall Kaspersky or TOTALLY disable it (preferrably uninstall) and test and let us know. Others have had similar issues with 3rd party protection. If the problem is Kasperky you can create an exception for WX5 to stop the false positive.

As for your old version, if you create a new project does it behave properly?

When updating to a new version WX5 offers to make a backup project, do you still have a backup?

(I always create an iwzip (step 5, export project) before any update/upgrade)

Dear, Kaspersky probably has nothing to do with it, because I installed it today, but the program hasn't worked for a week. I made another attempt, installed an older version and tried to create a project, it is also broken. I tried to import the older version of the project, it imported without problems and the project went to run, but the problem is the same, the page generation is broken.

I'm sending the last log

10.01.2022 10:19:29.561 [ERROR] Soubor C:\Program Files\WebSite X5 - Pro\Res\Gallery\02.png nebyl nalezen.
mscorlib
v System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
v System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
v System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share)
v WebsiteX5.BL.Project.Files.GalleryThumbGenerator.RunInput(Boolean uploadVersion)
v WebsiteX5.BL.Project.Files.FileGenerator.RunInput()
v WebsiteX5.BL.Core.FileCreation.Pipeline.Pipeline.doOneIo()

10.01.2022 10:19:29.657 [ERROR] Source file not found
(C:\Users\Admin\Documents\Incomedia\WebSite X5 - Pro\cestiny skutr 04_01_2021\Library\l12f8gm5vlidgk1m6w25ca2y4e5c14bf)
WebSiteX5.BL
v WebsiteX5.BL.Project.Files.CopyGenerator.RunOutput(Boolean uploadVersion)
v WebsiteX5.BL.Project.Files.FileGenerator.RunOutput()
v WebsiteX5.BL.Core.FileCreation.Pipeline.Pipeline.doOneIo()

Jiří, I suspect Stefano has offered the best solution, but you may wish to check the HD before doing anything. System IO error implies there may be difficulty reading the HD. If you intend to continue using this HD I would open a command prompt as administrator and  run chkdsk c: /r.  After this runs you can type sfc /scannow (from another admin cmd prompt) and try repair any system components that are missing or damaged.

In my opinion:

If you can afford it I would suggest a new SSD and do a clean install of Windows. It will be a new PC.

Thanks for the advice, but the result is the same. The repair did not detect any problem. Windows Resource Protection did not find any integrity violations. I think I'll try replacing the SSD.