I had to try a couple of times because pgAdmin refused to create the debug log in the same folder of the regular log.
I noticed another thing. When I press the "Backup" button pgAdmin seems to do nothing (you might think that you missed the button), and when I click Backup a second time is when the process crashes.
I tried leaving pgAdmin alone for a while but it gives no sign that is doing anything.
Set the log level to debug (Ref: https://www.pgadmin.org/faq/#8), Rename/Delete the old log file, restart pgAdmin4 and try to re-produce the issue again and then send us log file.