You probably should start another posting ... although I don't see a forum that appears to lend itself to the specific issue ... general help, maybe.
But, having said that ... am not familiar with your hosting environment, but 'blank screens' *could* mean the process is timing out. That's a guess. What are the settings for how long a script can run in PHP?
So, turn on debugging, upload users using your process and let's see if debugging kicks out anything at all.
In addition to that, check server logs access_log, error_log for the script that was being used ... off the top of my head, dunno what that would include but seem to re-call the word 'bulk' in there somewhere.
Those csv files are just text and should be small ... uhhh, how large are yours? So it's a processing thang, I think.
'spirit of sharing', Ken