I stumbled upon exactly the same problem today. Seems to be a bug. The reason for this error is that a type named “windows-vst” is simply not in the code. “vst” is though. If you compiled from source you can add a single line to route.cc, fire a “./waf” and everything will be fine.
I’m not sure if this is a dirty hack because I’m a lousy coder myself. If it’s not, it would be great to see it in svn.
Thanks to Mario’s fix and some help at Ardour IRC I have built an updated Ardour3 WinVST AV Linux package that should fix this issue for you and any other AV Linux 6 users of the WinVST build:
Get it here and please report if it works for you: