When filtering on dates in an OData filter you definitely don't want the single quotes. But the format of the date value being compared can be very finicky. I find that it is best to format the date and time being used as the standard ISO8601 format rather than a custom format. I tested this query in the list rows using the modifiedon date and it works as expected. As you can see there are no single quotes and the formatDateTime uses the default 'o' format for ISO8601.
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Paul Papanek Stork, MVP
Blog: https://www.dontpapanic.com/blog