Had this exact problem too. I checked 5V at PSU and was only 3.5V! This is a 550w OCZ powerstream I had used for years and just taken out of service to use with the hub because of it's 35A 5V rail. Found out the PSU needed a load on the 12V rail for the 5V to output correctly.
Don't know if the same as your problem but hope that helps. Ended up powering a BFL single along with the 49 port hub full of erupters.