Would you please run the following SQL “select owner, object_name, object_type from all_objects where object_name like ‘QUEST_SL_%’” in your database 10g and then give me the result?
Please try explain plan in SQL Plus with the same connection identities and see if you are able to get plan. If not success, try grant Select On the table at the back of the synonym ‘Quest_SL_Explain1’ and get plan again.