First thing I would do is try the backup option from x5.

It works for v13 evo as well.

https://helpcenter.websitex5.com/ru/post/47713

The Backup is to old. I have made a lot of changes the last days. 

But I backed up every day with the option "save project to disk". I did try to copy all  the files to the original project with error. But I still get the same error.

I will try the post in helpcenter 47713. And let you know

Then it's better to use one of your backup's.

Goto the folder where they are kept and import the created file.

I have now tried that. Renamed the 20171219xxxxx.iwprj2 to "project. iwpr" and keep both in the backup folder.

And it still does not work. Should it be "project.iwprj2"  ??

No, If you have made a copy in step5 with export project, then x5 creates 1 file with extension iwzip.

you can import that one in the project section using the import button.

What did you use? You should have used export the project, then you have made a good backup.

If you have used export the website to disk, you only have made a copy of the website, not of the project.

you need the project. I am not sure what you have used because you wrote: "save project to disk"

Hi again.

I managed to restore it  from the iwprj2 file. Just need to put it in the right library. 

In your  description it was written like  it should be in the same library as the backup files. You need to correct that for better understanding.

I know now, that I shall use "export project"

But thank you.