The default execution time in PHP is 30 seconds. Could be that the backup process is taking longer than that max. Try increasing max_execution_time in your PHP config to something higher ... 90 is a guess.
While you are at it ... might also increase:
max_input_vars > 1000
Change to php settings requires restart of apache/web service.
Then try the backup again - stepping through each screen ... not skipping to the end.
'spirit of sharing', Ken