HTTPS download through a SonicWall with SPI turned on fails

This has been bugging us for a month. We have a Windows 2000 server running IIS 5.0 hosting a number of HTTPS virtual folders allowing our clients to download and upload files to and from us for years. A new client has their server behind a SonicWall firewall and cannot download with HTTPS from our server. If they move the server outside the firewall they can get the files with no issues. If they turn off Stateful Packet Inspection on the SonicWall, they can get the files. If they simply dialup using any ISP dial up number, they can get the files.

The client has claimed that they can get files via HTTPS from other sites with no issues, so it must be our issue and has not been willing to work with us further until we fix the issue. We have over 100 clients transfering files this way everyday with no issues.

I cannot get support from SonicWall as I do not have an active contract with them and their knowledge base is no help here. Further searches have yeilded no results either.

Does anyone have any ideas?