"Sorry we couldn't find File ABC" upon cut/paste to different folder

The issue you're experiencing may be due to a limitation of the applications that you're using to open or save the files in question OferL - this is something compeletely out of our control. In short, there are files that your applications can't process correctly because they have file paths that are too long. 

This behavior is present if there are more than 255 characters in the entire file path. This means from the C:\ all the way to the extension of a file.

As an example if you had a test.txt file in your Dropbox that had the following path:

 C:\Users\OferL\My Documents\Dropbox\test.txt

This file would have a path that is 47 characters long while if you moved the same file to your desktop directly as you mentioned you'd be immediately reducing its filepath to fewer characters as it would result to something like: 

C:\Users\OferL\test.txt

Based on your example now, when you move this copy to your desktop as you said, you're immediately reducing the number of characters on the filepath and this is why you're able to open it without issues.  

Keep in mind that our desktop application can read and write to locations that are longer than 255 characters, which means that files will sync to and from those locations, however, the applications used to open or save said files (e.g. Word, Notepad, Excel, Etc.) might not support reading from or writing to these locations with long file paths and may show error messages when trying to open, save or edit them. This is out of our control since the application developers would have to add support for long paths to resolve the issue.

You should be able to resolve the issues with your applications processing these files by simply renaming the file to a shorter name, renaming a folder to a shorter name, or reducing the number of sub-folders.

I hope this clarify matters to you and please let me know if there's anything else I can help with.