Did you use the same pubkey for 0xChat and npub.cash?
I managed by copying the minted e-cash in epub.cash as a clipboard and went back to 0xChat wallet. The pop-up then appeared. I log in in 0xChat with Amber.
You just need to enter the 0xChat wallet, and it will automatically check the balance of npub.cash. It has nothing to do with the clipboard