@Alan_Cheung1 sounds like the chat memory is sending back a message format v4 rejects on turn 2, drop the memory node and reconnect a fresh Window Buffer Memory with context window set to 1, or just swap the deepseek node for a plain HTTP Request to https://api.deepseek.com/chat/completions so you control the body shape yourself.
@AlanCheung1 deepseek v4 is stricter on function schemas, in your tool node make sure every param has a type and description and drop any nested objects, also try setting toolchoice to auto in the model node options, v4 chokes when the schema has loose types that v3 silently accepted.
It happens sometime, same for the postges, its not your fault. Have you tried the deepseek v4 from the openrouter? If it still happen try contacting the n8n support