This is an old problem, there is even a TOAD related answer on StackOverflow
When the table TOAD_PLAN_TABLE is dropped, Explain Plan raises
[Error] Explain Plan (3: 68): ORA-01775: looping chain of synonyms
This cannot be resolved without dropping the synonym manually
DROP PUBLIC SYNONYM toad_plan_table;
the next call to Explain Plan raises
[Error] Explain Plan (2: 57): ORA-02404: specified plan table not found
Since the solution is less than obvious because of the Oracle error message, is there a way for TOAD to handle this better?