It’s been a while since you started this thread, so the situation may have changed, but originally, this was the problem:
The SYNONYM was invalid (according to the STATUS column in DBA_OBJECTS), but the target object was valid.
Toad’s health check reports all invalid objects (including synonyms) but when you click on a synonym in the Schema Browser, it will show you the target object on the right hand side.
Apparently, it is possible for Oracle to mark the synonym as invalid, even though the target object is valid. I think you can safely ignore it.