@liemph Thanks for the feedback and i hope my solution helps!
I had asked the M5Stack support for that problem and they wrote me "The PSRAM was soldered on to the board. Cant disable it." That implies in my understandig that the Port C cannot used since the PSRAM uses the same pins as Port C. And looking closer to the M5Stack support-replies above shows no contradicition to my assumes but only some unclear statements and regrets.
For me is no doubt that the Port C of the "Fire" is a kind of rudiment of earlier M5Stack versions. The Port is not usable in the "Fire".
Ok, I have to give up or unsoldering the pin which I certainly will not do it.
My decision of buying the FIRE version of M5Stack core was based partly on the PSRAM. I think it is a faulty design.