ORA-28040: No matching authentication protocol (when connecting from 11.2 client to 12c server)

I gave that said database I received from our customer to avail for my colleague and he came back with an error he got:

ORA-28040

I tested explicitly from an 11_2 instantclient using sqlplus and it worked. In his configuration he was using SQLTALK (Gupta) and their Oracle gateway, which we are still using over decades now, normally without problems.

But the precence of different clients (32/64 bit) may cause problems and I'm suspecting sth. in this vein.

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Christoph