Hi, today I have again exactly the same problem unfortenately. My coldcard cannot read the PSBT file. If I press "ready to sign" it says "Coldcard is ready to sign". I contacted the Coinkite support and they suggest using another SD card, but this SD card I ordered directly from them. So I don't think this it the issue. I also followed all steps from the Coldcard website, their manual. Also it doesn't matter if I turn the coldcard on before inserting the SD card, or if I first import the SD card and then turn on the Coldcard.
Anyone has some ideas still? I'm thinking about purchasing a totally other wallet, like foundation passport, and inserting the coldcard seedphrase there. Getting quite frustrated with coldcard.