For me the difference between WARN and ERROR is in how likely you need to take an action if a error happens:

WARN - something is off a happy path but it's either out of you control (external system sent a broken request) or not urgent (e. g. use of a deprecated API) or should be handled gracefully (timeout for a sub-request with multiple retries, a request may still succeed but with high response time)

ERROR - you need to take action or at least may need - e. g. indication of a bug or a problem in the infrastructure (cannot connect to a database or disk is full, or a process has run out of file descriptors, an expired certificate, invalid auth token e. t. c.).

Of course one can lamp two these levels together but it makes Ops work harder.