When an event registration levels are set up and associated with specific fees, users can also list costs for those registration levels. That way, the receipt amount that the registrant receives is less than the full registration cost, to cover event costs. When a registration level is set up with no associated costs and the receipt amount should equal the full registration amount though, the receipt amount for any payments applied to the event registration defaults to $0.00 instead of showing the correct receipt amount.
This issue is similar to Knowledgebase solution BB729671, but that solution was created for event registrations that included associated costs. If there are costs associated with the registration type, and the receipt amount is already supposed to be less than the full registration fee, then the receipt amount will default correctly. The problem now only occurs when the receipt amount is supposed to match the registration fee amount.
Download and install the latest patch, which contains all fixes from previous patches. If you are running an older version, download and install the latest version and then the patch.