I recently ran into this error while trying to connect to a new Oracle database using TOAD. The following steps seem to resolve it…
step 2 – restart TOAD and try connecting using the new tnsentry you just created
Strangely the first time after adding the new entry, I was unable to see the entry in the connection dropdown. So, I just manually typed in the new tnsentry in the Database textbox at the TOAD login prompt and voila!
I have TOAD 10.5.0.41 on my PC and there are probably patches available for this version of TOAD that I should first install.