Sometimes getting blank response to api

Occasionally I get an almost empty page as a response to the api, when adding events. Usually I just get this one line:

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It happens 5-10% of the time. It's quite possibly a data issue on my part, but I don't get any error response so I don't know what I'm doing wrong. I can give you sample data but I was wondering if there was a general answer as to why this was happening.

Thanks very much,
Nancy