Flex "Internal build error"

I started getting "Internal build error" messages whenever I build anything other than a one class Flex library. The log error from C:\work\workspace\.metadata\.log is:

!ENTRY org.eclipse.ui 4 0 2008-03-29 09:13:16.625
!MESSAGE Unhandled event loop exception
!STACK 0
java.lang.NullPointerException
at com.adobe.flexbuilder.exportimport.importwizard.FolderProjectImporter.findFile(FolderProjectImporter.java:107)
at com.adobe.flexbuilder.exportimport.importwizard.FolderProjectImporter.computeDeepPath(FolderProjectImporter.java:95)
at com.adobe.flexbuilder.exportimport.importwizard.FolderProjectImporter.validate(FolderProjectImporter.java:41)
at com.adobe.flexbuilder.exportimport.importwizard.ImportWizardPage.validatePage(ImportWizardPage.java:310)
at com.adobe.flexbuilder.exportimport.importwizard.ImportWizardPage$4.modifyText(ImportWizardPage.java:178)
at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:166)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:66)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:938)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:962)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:943)
at org.eclipse.swt.widgets.Text.wmCommandChild(Text.java:2366)
at org.eclipse.swt.widgets.Control.WM_COMMAND(Control.java:3801)
at org.eclipse.swt.widgets.Control.windowProc(Control.java:3675)
at org.eclipse.swt.widgets.Display.windowProc(Display.java:4351)
at org.eclipse.swt.internal.win32.OS.CallWindowProcW(Native Method)
at org.eclipse.swt.internal.win32.OS.CallWindowProc(OS.java:2179)

I found a recommendation to re-install Flex Builder, so I've done that and got a "Product Extension not established automatically" error. Jeez. More detail:

java.io.FileNotFoundException: C:\Program Files\Adobe\Flex Builder 3 Plug-in\eclipse\features\com.adobe.flexbuilder.feature.core_3.0.194161\feature.xml (The system cannot find the path specified)

Clicked OK to let it finish anyway.

Back to Eclipse and...success! The project builds properly!