I had that happen when I was using QB desktop.

I had to log into my email provider

change my password

go back to qb and update to new password

And then it worked.  you could always do the same steps above to put the old password back in.

Resolution for 'app security issue: 'quickbooks email does not accept my gmail password tried changing app security with gmail and it still will not work' available: Yes.

The app security 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 app security issue yourself by looking for a resolution described in the article. If it is a complex app security issue or you are unable to solve the issue, you may contact us by clicking here or by using other app security support options.

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