Why do I get, “The size necessary to buffer the XM

Question

Based on what I see here, it seems as if "The size necessary to buffer the XML content exceeded the buffer quota" is usually due to a problem in the .rdl file, rather than really being an excess of data or so.

But when there's a problem with the contents of the .rdl, usually the Preview tab won't display at all, instead informing me that errors in the report first need to be fixed.

I'm getting no errors, though; it allows me to enter the report parameters, and runs for a time generating the report; only then does it pop up the "The size necessary to buffer the XML content exceeded the buffer quota" on the screen:

What could be causing this, whether the problem given is valid or it could be something else?

UPDATE

The "simple way to solve it" according to Kim Crosser at the link above (repeated for your clicking ease here) didn't do it for me - I was able to upload the file to my report server with no err msgs, but on running it I eventually got:

The remote server returned an error: (500) Internal Server Error.

Does this err msg, similar to the "exceeded buffer quota" msg actually disguise the true reason for the failure? Because it sounds like an external problem rather than something specific to my report, but I have my doubts that it isn't something in my rdl code...