When logging into Perusall, the page might become blank if adblockers or antivirus software is interferring. This can be easily evaded if you disable the blocker or software for Perusall. Typically, it will display a pop up on the page stating that it's blocked for your protection in one variation or another.  

1. For BitDefender or any other blocker, navigate to and select "go to the Perusall page anyway". It could be another variation of wording. Make sure you give the device permission to access Perusall and everything should run smoothly.