What is your Oracle server version?
DBeaver uses query "SELECT U.* FROM SYS.ALL_USERS U WHERE (U.USERNAME IN (SELECT DISTINCT OWNER FROM SYS.ALL_OBJECTS))" to read schema list. Does this query return any results in your case?
You can enable metadata logging in query manager (Preferences->Database->Query Manager, enable all object types and query types). And then check how many rows each metadata query returns.