It could be the browser release. Did 6.5.1 have GZIP compression is on by default? We went from 6.0.2 to 6.5.2 and had problems with the various browser levels.
In IE 6.0 support for GZIP is broken. You need to have Service Pack 1 on at least (6.0.2800… is what the version will show as I believe).
There’s some IE fixes that seem to have an impact on this too. we ended up disabling GZIP on the server via the NOTES.INI for now.