Qlik doesn't execute any sql-code else it just transferred the statement per driver to the database and waits then to get a return. If you now get an error it's caused from an unsuitable driver and/or a wrong connection and/or missing user/access-rights to the database/network or something similar.
I think I would start with something simple, like:
sql select KnownField from KnownTable;
to ensure that the access in general worked - and if it worked you could go further eliminating step by step all other possible causes.