This alert is caused by the undefined text format of the footer in the views signup_available_signups and signup_current_signups. Open the view, select under No 'results behavior' the footer text and select the text format 'plain text', save the view.
Other modules than signup had comparable problems with coded views.
Closed: duplicate
1.0
Views integration
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
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