From the vBTubePRO Admin:
Admin Panel vBTubePRO 1.1.0 Beta_5
PHP Config Status:
Uploads: 1 | Max Filesize: 200M | Memory Limit: 32M | Max Post Size: 200M | Max Execution Time: 30
From the vBulletin Admin:
Server Type Linux
Web Server Apache v2.0.63 (cgi-fcgi)
PHP 5.2.6
PHP Max Post Size 200.00 MB
PHP Maximum Upload Size 200.00 MB
PHP Memory Limit 64.00 MB
MySQL Version 5.0.51a-community
MySQL Packet Size 100.00 MB
Strange - since i first posted about this, an upload of 23MB .wmv was perfect, at ~50k/s upload speed, but then a 21MB .wmv encoded from the same source and on the same encoder gave the error

So, it's not the filetype. Also: I can convert the files in a terminal with tovid easily here, so the files are not corrupt.
Also: error happend from here using Opera on Linux (KDE), so forget the IE/XP details.