MarketPress CSV Importer Throwing 504 error

Just to test the CSV importer, I downloaded the sample file, removed all but 1 product and re-uploaded. When I did this, it threw a 504 Gateway Time-out error and then I could not access the admin area, or the site for several minutes -- it said there was an internal server configuration problem (500).

This was on a child site of an MP installation. The main site was not affected and the child site eventually returned to "normal" -- albeit without the uploaded products.

Any thoughts on how I can fix this?