One of my clients reports that a project tracking file I created for them (years ago) is now giving a runtime error 440 (Automation error) when they try to add a new project.

The file has two userforms: NewProjectForm (for adding a new project) and EditProjectForm (for editing an existing project).
The runtime error occurs when executing the NewProjectForm.Show command. That userform has a UserForm_Initialize routine that initializes several comboboxes, checkboxes, and textboxes. The userform is never displayed; the error occurs before the form is displayed.
They have tried the file on at least two different PCs in their office - same result on each.

I am puzzled by two things:
-- The file works perfectly fine (no errors) on my PC, and has worked fine for them for years (until about a week ago, apparently).
-- The EditProjectForm ALSO has a UserForm_Initialize routine that initializes several comboboxes, checkboxes, and textboxes, but THAT userform works fine for them (no errors).

Can anyone suggest how to troubleshoot this? What could have changed in their system to cause this Automation error? Why would one userform crash, but the other one works OK?