For what it's worth. I was having the same problem and had tried everything I could think of when it occurred me that perhaps it was something with my browser. Even though I had not experienced this before with Safari, I tried using Firefox and was quite surprised to have paypal process the payment ok without error code 50042. I am preparing to upgrade to OS X Lion and have been receiving downloads from Apple to get some things ready for Lion. I know that one of the updates was to Safari so maybe that has something to do with it. Who knows what's going on, but maybe this will give you something to think about.
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