I have read a few posts on this forum describing problems with opening the editor with JSP files, but none of them seem to apply to the problem I have.

First of all, here is my configuration information:
Eclipse: Version: 3.3.0
MyEclipse: 6.0.1 (from MyEclipse_6.0.1GA_E3.3.1_Installer.exe)
JDK: 1.5.0_09

Second. I have been using this configuration without a problem for nearly two months so I know these version work together. I have not touched the configuration or installed anything new on my laptop since then. Yesterday during the day, I was successfully able to open and edit JSP files, but last night when I started MyEclipse, I suddenly started getting errors. First I get a popup window that says: "Error opening the editor. (Time of error:...) Reason: Argument not valid." Then an editor window appears and I click on the "Details" button, I see the stack trace below.

This morning, I uninstalled MyEclipse and was able to edit my JSP files once again, but when i re-installed MyEclipse, the errors returned.

Does anyone have any suggestions?

[partial stack trace]
java.lang.IllegalArgumentException: Argument not valid
at org.eclipse.swt.SWT.error(SWT.java:3547)
at org.eclipse.swt.SWT.error(SWT.java:3481)
at org.eclipse.swt.SWT.error(SWT.java:3452)
at org.eclipse.swt.custom.SashForm.setWeights(SashForm.java:374)
at com.genuitec.eclipse.webdesigner3.editors.WebDesignerMultiPageEditor.configureSash(WebDesignerMultiPageEditor.java:637)
at com.genuitec.eclipse.webdesigner3.editors.WebDesignerMultiPageEditor.createDesignPage(WebDesignerMultiPageEditor.java:573)
at com.genuitec.eclipse.webdesigner3.editors.WebDesignerMultiPageEditor.createPages(WebDesignerMultiPageEditor.java:340)
at org.eclipse.ui.part.MultiPageEditorPart.createPartControl(MultiPageEditorPart.java:283)

Can you switch to a new workspace by going to File > Switch Workspace > Other and test this out again ? Sounds like your workspace has gone corrupt.

Great suggestion. I only use a single workspace, so I first created a new workspace and then I imported an existing project from the old workspace. The editor does open without error in the new workspace! Thanks!

The question remains as to how the original workspace became corrupt. The only thing I can think of was that I had the laptop session open for 10-14 days (using hibernate on occasion) and it might have been that long since I closed the Eclipse application. The problem did start after I shutdown the application (and computer) and started it back up later that evening.

Thanks again.