FMG cannot install policy due to "unresolved objects"

Hello,

For some reason my FMG fails Policy and Object validation and cannot install policy changes any more. It responds with:

"The following policies will be skipped due to unresolved objects" without telling me which objects are unresolved.

I can bypass the problem with Re-install policy as a workaround, but I have to resolve the issue with install. Do you know why am I getting this message and what I can do to fix it?

Thanks