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The problem seems to be... outputs('Compose') is null i.e. empty.
So you need to check which value is currently assigned to your 'Compose' action block.Assuming you assigned a Forms response to it, it means somebody did not answer the question, and you get a null value instead of a date modelled as a string in ISO 8601 format.
So, either you mark the question as mandatory, or you add extra logic to replace null and handle a default date instead.
Hope this helps
Yes, the issue was traced to the Microsoft Forms server.
All fields on the form are required (not optional). Downloading the Excel sheet with the response data, those entries are gone. There are 17 response IDs missing.