If you get an error message in your Proxy Plus error log like "sent a HTTP request header containing a line longer than allowed limit (>4096 chars)." then you have hit a security limit.

I note this issue occurs when using Windows Live in Microsoft sites via the proxy server.

The fix is to use regedit by adding HTTPLineMaxSize with type DWORD and value larger than 4096 (decimal) into HKEY_LOCAL_MACHINE\Software\Fortech\ProxyPlus\Security

After you restart P+ it will use new maximum for length pf header
lines.

This information was sourced from http://www.proxyplus.cz/viewmsg.php?lang=en&id=7573

FYI