OK, this has nothing to do with the error's description, but nevertheless....
If you're getting this in a Lasso inline, the cause is too many requests from Lasso hitting FileMaker quicker than Web Publishing Engine can handle them. A great solution is to insert [Sleep: 50] or some other value, before each inline request. This will give the previous FileMaker request time to finish up before starting the new one. Works great.
Another symptom that you'll see is that the page takes unusually long to process, and one or more of your inlines on the page will fail.
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