Question mark icon

Out of the blue there appear randomly question mark icons in the path panel. I have checked everything; the path is still working, all pictures are there, nothing was altered and the catalog works as it should do. Seem s to be no obvious reason for these question marks. Nevertheless I am always worried aboud error messages and would like to know, why they appear.

Any ideas, what's going on?

I'm on Windows, Media Pro version 1.5, pictures and catalog files are on a company network.