Your file could be getting corrupted during the FTP transfer. Though this isn’t the most likely scenario, it isn’t unheard of though. Make sure your transfer mode is set to binary (most ftp clients can auto-determine the setting nowadays), and not ASCII mode. Again, not the most likely thing, but something to look at.
Also, try tweaking the export to WMV a bit. You’re obviously already dropped to 15fps; try encoding as VBR instead of CBR.
Lastly, your webserver could be botching the actual serving-up of the file. Again, not the most likely, but may be worth sending a nice email to your host in case you’ve found no other remedy.
Good Luck.
-Mark