In a merge replication topology over the Internet, when you browse https://host/app_pool/replisapi.dll through an ISA server, you may receive the following error message:
Error Code 64: Host not available
Replisapi.dll returns an HTML page that is intended just for testing purposes. The HTTP response that delivers this HTML message is not complete. The ISA server interprets this as an invalid response and closes the connection resulting in the error reported in the Symptoms section.
When testing your web synchronization topology in these environments, use https://host/app_pool/replisapi.dll?diag.
This issue does not cause any synchronization problems.