You are migrating your SEP clients from an older release to a newer one via the auto-upgrade feature, but it fails.
Error 404 appears in the sylink.log and error 404.3 in the server's IIS log when the clients attempt to download the delta package from the management server (SEPM).
404.3 - MIME Map policy prevents this request. This problem occurs if the following conditions are true: 1. The handler mapping for the requested file name extension is not configured. 2. The appropriate MIME type is not configured for the Web site or for the application. It can happen when the Symantec virtual folders have been moved in a new web site under IIS but the required file name extensions are not put in place.
You have to check if the required name extensions are in place:
Open the Properties of the Symantec Web site in use
Select the HTTP Headers and click on the MIME Types button
Check if the following file name extensions are in place: .bat, .cfg, .dat, .dax, .db, .grd, .hst, .ini, .kc, .loc, .manifest, .plg, .policy, .reg, .rul, .sig, .spm, .sys, .wlt.
Click OK and apply the changes to the virtual folders as well.