library webutil cannot be found

we are setting up a new Oracle 12c application server environment on Windows 2016.

on running a report, we got that error message, even if report is finally created. So, we checked out our report, removed all attached libraries and recompiled it; but error message is still appearing.

I checked out REPORTS_PATH key on registry, and webutil.cfg file, and they look like to be the same as the ones in test environment, which works correctly. But above all, why does report search at runtime for a library that is not included in its source? whats wrong?

thanks for help