We had (and still have) the same issue, its not related to proxy issues but duplicator failing to progress to step 2 in the installation routine.
We narrowed it down to the point where after unzipping and changing permissions the installer would progress to step 2, but after the redirect and check of status file he thinks step 1 was not completed.
Actually, when this is done the first time, the step 1 never “really” completed, it is stuck showing a progress bar (that is when the proxy just says 50x Gateway error cause the response times out). Looking at the logs, it says step 1 complete so it must be some problem with the postback in step1.
I’ve seen similar issues reported here, for us the issue appeared after upgrading WP to 5.x and upgrading Duplicator to latest.