TMG 2010 SP2: The HTTP message includes an unsupported header or an unsupported combination of headers. (12156)

I'm seeing an error message when requesting a web page that is processed by a TMG firewall rule:

Error Code: 500 Internal Server Error. The HTTP message includes an unsupported header or an unsupported combination of headers. (12156)

Another forum post (TMG 2010: 12156 The HTTP message includes an unsupported header or an unsupported combination of headers) suggested thatKB 935693 is still relevant for TMG 2010. However, when I went to add the registry key value mentioned in the KB article:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\RAT\Stingray\Debug\Web Filters\DROP_CONTINUATION_LINES : 0x00000001

I discovered that the following key/value already existed:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\RAT\Stingray\Debug\Web Filters\APPEND_CONTINUATION_LINES : 0x00000001

I am guessing that APPEND_CONTINUATION_LINES would append any header lines beginning with a whitespace character to the previous header line, and therefore DROP_CONTINUATION_LINES would not be needed. However, both with and without DROP_CONTINUATION_LINES I continue to see the error message.