@JuliaMikkaelaQ 

RE: Did they try using the verification code you received? Were there any error messages or prompts?

You think that QBO, whilst being all super-secure, is going to accept a validation code of "0"? That'd be amazing!

RE: Sometimes, such unusual circumstances are caused by browser-related matters. If the system generates a valid code, ...

Yea, but it didn't. Why not address the actual issue instead of going on about browsers and other unrelated things?