Did your JuiceboxBuilder-Pro installation always do this or did it work when you first installed it?
If this has only just happened recently, has anything changed on your computer (any software updates) which may be contributing to the problem?
Try installing and running JuiceboxBuilder-Lite which comes bundled with Juicebox-Lite and can be downloaded from this web page.
(You can install and run JuiceboxBuilder-Lite and JuiceboxBuilder-Pro side-by-side on the same computer.)
If the same problem happens immediately with JuiceboxBuilder-Lite, then this suggests that there is something on your computer which is preventing both applications from running correctly.
If the same problem does not happen immediately with JuiceboxBuilder-Lite, then this suggests that there is something specifically wrong with your JuiceboxBuilder-Pro installation, although if this is the case, I would have expected a completely clean installation (including manually deleting all related files before re-installation) to have solved the problem.
I hope this at least points you in the right direction.