Debugging structures can be a bit problematic sometimes with lazarus. I rather skip it. I do most my work with fpc.
It can't hurt to at least write out some lines sometimes
I think your problem might be that when the stream is read with html data that its position is set to the end of the stream. Therefor parsing it into a htmldocument delivers nothing at all.
I do not have experience with synapse though, so am assuming that HTTPSender.Document is a stream and faces the same issue ?