Validation failed – even when no changes are made

I have used Events Manager for several years. Monthly meetings are present as recurring events. When an agenda is available I edit an individual event and link to a pdf in the text box. This has always worked fine. Now when I make that change I get a message to say 1 field failed validation. Even if I remove the pdf link it still fails.
How do I know which field is failing validation and why does it fail when nothing has changed?

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