Acid Pro 10's handling of VST plugins are troublesome, especially VST3 plugins.

Did you notice if it's hanging onto a specific plugin during the scan? These typically the best way to identify the plugin that is some how causing Acid grief. It sounds like that is where your particular trouble is occurring, where you will likely need to remove that plugin out from your VST plugin folder.

What I noticed is that when Acid stumbles on a certain plugin during startup, it will typically throw up a black pop-up which gives a couple prompts of "Skip plugin", or "Stop scanning". However, I often find if I just wait and do nothing, it finally is able to load it and continues on but it does seem to take a couple minutes in some instances.

Is it possible that box is hiding behind Acid during the start up and you're not seeing it?

What you can do is that I believe by default Acid is setup to scan the C:\Program Files\Vstplugins folder. You can temporarily rename that folder to "VSTPlugins_Temp", so Acid will no longer scan it during startup and therefore you can determine if it's a bad plugin preventing Acid from starting.