It means that the program cannot import the compiled dlls in some reason. You can easily fix that by deleting the precompiled dlls from the Library folder at:

C:\Program Files\Forex Strategy Builder Pro\User Files\Libraries

On the next start FSB will recompile the indicators again.

...

I have to find a way to intercept such messages and to make them more readable.

Did you try running FSB Pro with empty Libraries folder?

Actually it must delete the dll files alone in such cases.

The only thing coming to my mind is to delete the "Libraries.json" files from the System folder and to tray again.

No more ideas. 
Only guessing that a FSB reinstall or windows restart may help. If not, you may try to find what exactly indicator makes the problem.

I have experienced such problems when FSB Pro tries to load a dll security signed from the legacy FSB or from other FSB Pro version.

Thinking in that direction, you have a developers license that do not require signature for the dlls. Are you copy of FSB validated? If not, it may not load the dll compiled from your validated copy. However, all this doesn't make sense when the dlls are deleted. Is it possible to have other custom location of the user files where to have indicator dlls? You can easily see that when open File->User Files from a running program.