It seems the extra web security feature (mod_security) is configured to return HTTP 503 when the user agent is "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)"
I contacted support and was told "IE6 is not supported" line, repeatedly. I completely agree IE6 is an old browser and no one should be using it, but some people still are. It is an issue between my site and my visitors, not the hosting provider.
I would understand if my question had been "Dreamhost panel does not work properly in IE6".
To test the "not supported" argument I tested IE5.5, IE5, IE4, IE3 etc... listed below. So it would seem if disabling older browsers improves security then dreamhost is failing, badly. Esp. as only 1 of the 50+ variants of the user agent string used by IE6 are blocked.
[*]Mozilla/4.0 (MSIE 6.0; Windows NT 5.0) - another IE6 UA variant
[*]Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1) (IE6 on XP without
service pack 1)
[*]Mozilla/4.0 (compatible; MSIE 5.5;)
[*]Mozilla/4.0 (compatible; MSIE 5.00; Windows 98)
[*]Mozilla/4.0 (Compatible; MSIE 4.0)
[*]Mozilla/1.22 (compatible; MSIE 2.0; Windows 3.1)
[*]Dr Horse Evil Botnet attack node 0x5847