Compare & Deploy Metadata - Error 400 Unable to forward request due to: Read timed out
You may notice that when deploying, when you reach the Compare & Deploy Metadata feature and try to perform a difference comparison of metadata, the result errors out.
Hover over the word "error" for more information about it. You will see that the error message is:
“Error 400 Unable to forward request due to: Read timed out:”
If this is the case, there is a limit of 30 seconds that the Heroku platform waits to get the differences. If that time limit is exceeded, the error is returned and you will have to retry the action again (by clicking on the word "error" again). The error you are seeing is likely due to the time exceeding the 30 second limit.