Photoshop :: Error 22 / Window Has No Constructor

Using the ExtendScript Toolkit, it was clear that when photoshop loaded, ScriptUI was not getting created - any of the objects associated with ScriptUI returned undefined in the console, but the rest of ExtendScript was up and running. Even more odd was that these objects DID exist right up until you started Photoshop.
 
Turns out that our CS5  install has TWO copies of the ScriptingSupport.8li plug in - one in /Plug-ins/Extensions the other in /Plug-ins/Automate. Photoshop seems fine with multiple copies of a plugin, and only loads one of them. However, only the plugin in /Extensions has the additional ScriptUIFlexServer.swf, ScriptUIFlexServer-app.xml and ScriptUIFlexPhotoshop.swf files alongside it that appear to be necessary for ScriptUI to load properly. This appears to be a historical side-effect of the way we install CS, but it's a pretty easy situation to get into.
 
So, when Photoshop starts it chooses which of the plugins to load and if it loads the one in /Automate, ScriptUI is broken and anything that tries to open a window fails with Error 22.However, photoshop also caches the plugin locations, so once it's loaded the wrong one, it always does so, until you trash the preferences, whereupon it chooses which one to load again.This is why deleting the prefs works most of the time, even for our installation.
 
The real solution is to make sure you have only one plugin that has the ScriptUI files with it For us, this meant deleting ScriptingSupport.8li from /Plug-ins/Automate, but you might find you need to reinstall the ScriptUI files.

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