Thank you. I've already tried that by executing GRANT SELECT ON SCHEMA XY TO ROLENAME, but the desired user still cannot select any tables from that schema. In the security tab and the role profile the result is an object privilege with SELECT permissions, so that should be good.
Even when I right-click on the schema and perform a find table request it won't find anything.
When I try to select the tables by using SQL it says table not found (but the table is definitely there).