I found one problem in 6.4c. After updated to 6.4d the problem was solved.
I found the same problem in differents customers even with 6.4d. To solve the inconsistency I requested customers to clear browser cache. It is a very, very critical solution.
I change for each VWG release the "PrivateVersion" of my application to force client update, but I think doesn't work.
I think that "StaticResources" ignores "PrivateVersion" changes and don't sends updated kernel.
The VWG configuration :
<StaticResources Mode="On" />
<PrivateVersion Value="188" />
<IconsPreloading Mode="On" />
<ShowLGPLWaiver Mode="Off" />
<UseFlashForUpload Mode="Off" />
<InlineWindows Mode="On" />
<GZipCompression Mode="On" />
<SkinsPreloading Mode="On" />