It is definitely a cookie issue.  Here is a post I created (it happened to me and a couple of clients–I had to do this for them).  In the post it steps you thru how […]

Resolution for '502 Bad Gateway issue: '502 Bad Gateway' available: Yes.

The 502 Bad Gateway issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the 502 Bad Gateway issue yourself by looking for a resolution described in the article. If it is a complex 502 Bad Gateway issue or you are unable to solve the issue, you may contact us by clicking here or by using other 502 Bad Gateway support options.

For more information about resolving issues related to 502 Bad Gateway in Quickbooks/QBO or to get support for the issue '502 Bad Gateway', click the link below: