There are a number of possible causes for being unable to download a TFE support bundle through the installer dashboard. If running the CLI command produces a small bundle (under 1gb) that is timing out in the dashboard and there is an ALB in the instance, then the problem may be the ALB.
The problem in the UI is a failure to download a TFE Support Bundle.
The default timeout for an ALB is only 60 seconds. This may or may not be enough time for your instance. Additionally, if the timeout was adjusted lower than 60 seconds it will almost certainly fail.
Significantly raise the timeout on the ALB and then attempt to download the bundle through the installer dashboard. If the ALB was the problem, it should be resolved. If you are unable to download a support bundle after adjusting the timeout, please contact HashiCorp support.