If you ever receive error code 6190 when trying to use FM Books Connector Online, you may want to verify the status of your QBO account and check to see if the account status is set to EXPIRED or SUSPENDED. According to the following article, an account that is either Expired or Suspended will throw error code 6190 when trying to do anything outside of a read operation.

Here’s the link to the article:
https://developer.intuit.com/docs/0100_quickbooks_online/0300_references/0000_programming_guide/quickbooks_online_subscription_states