Here are the un-BASE64'd auth values being sent to the XMPP server.

In Windows:

authzid="bobthebuilder@example.com",charset=utf-8,cnonce="xxxxx1e3552e
+9ea806f6411931b62b25",digest-uri="/",nc=00000001,nonce="xxxxx0dyy8ivm
+M8JcDfy44rlPAvDprBeLWHfW4Ea",qop=auth,realm="example.com",response=xx
+xxxd5b82f6faf7ed2feebcb70ba974,username="bobthebuilder"
[download]

In Linux:

authzid="bobthebuilder@example.com",charset=utf-8,cnonce="xxxxx895143c
+dc97d8d8eada76acc50a",digest-uri="xmpp/example.com",nc=00000001,nonce
+="xxxxx2YLz+OufKiUQ0zSBkaO1ntzY/Cpwg6+ns+l",qop=auth,realm="example.c
+om",response=xxxxx7d0924f2093e95bc0da1b136092,username="bobthebuilder
+"
[download]

It seems the Windows version is missing the digest-uri value. I'm going to bet that my problem is there.

Changing AuthSend to AuthIQAuth makes it work fine. This disables SASL. I'm not happy about it. Possibly TLS will take care of that? I'm going to find out.