FYI -- I just came here searching for "Blank Pages". I started having this exact problem 2 days ago, tearing my hair out trying to figure out what the trouble is.
The only "custom" portal box items I have are a PayPal button and an AddThis bookmark button, and both have been running forever on 7.2.2 with no trouble. the blank page thing started spontaneously, whith no changes being made by me on my dedicated server.
I upgraded to 7.3.0 last night, hoping that would fix things, but sadly, not.
I followed the advice you gave here about deleting files from /sessions and /templates/complie, but that trick didn't work either.
My users and staff are complaining, and my daily post count is dropping dramatically as users back away in horror from the white screen of death.
I swear -- I changed **NOTHING**. It began on its own.
