This editor cannot be displayed due to errors in the source file

Summary:

VBS giving error when trying load page.

Content (please ensure you mask any confidential information):

Created a brand-new project on Friday. Amended pages for Time and Labour and Absence pages and published. All worked okay.
After publishing, amended business rules for the Employment Info page all worked okay. This morning I previewed the page to demo to a colleague. All worked okay.
Later, went to advanced mode to check setting, but changed nothing.
Went back in the page later in the day having change nothing and now get “This editor cannot be displayed due to errors in the source file”.