The issue is most likely that it is timing out. Every field that is submitted means up to 4 and sometimes more DB queries. I find this happening on my sites mostly when there are complex repeater or flexible content fields. But if you have enough fields it could potentially happen even without using these types of field. A browser will only wait for 30 seconds for a request to send a response then it times out and results in a blank page.
The process is still going on the server and everything will happen, but the user will not know it.
Unfortunately there is not easy fix and your best bet is to reduce the complexity of the form. Maybe provide more than one form, but I don’t know how that would work.
