Hi Melvyn
I faced this problem today. I was installing the new release of our application (PB executable) on our two citrix servers. After installation, it worked but on one server but on other one it thrown the error "Unable to find pbvm125.dll"
And we, uninstalled the new release of application, reinstalled the old release, uninstalled old release and installed the new release again.
We actually still don't know what could be the real cause but doing the above fixed our problem.