502 HTTP_STATUS_BAD_GATEWAY

I am using Merge Replication between SQL Mobile clients and SQL Server
2000. This all works over an https connection using IIS 6.0.

This works everywhere except in one location where a client uses a
Proxy Server for all internet traffic, when on initiating merge
replication they are presented with error:

502 HTTP_STATUS_BAD_GATEWAY ("The server, while acting as a gateway or
proxy, received an invalid response from the upstream server it
accessed in attempting to fulfill the request.).

the URL for replication is of the form: https://.....:1234/../sqlcesa30.dll

Anybody have any ideas on how to fix this problem?

Joseph