Why comparing strings as it is a date in the first place?
Are you sure the that all the holidays in the list have a valid date?
You probably have an error as there are data differences between your local environment and your deployed environment.
The HolidayDate column will have a null or empty value. Already in this case the backend microflow should be checked.
You can set a default db Date like '01/01/1900 00:00:000' and it will solve your problem.
If your situation is somehow unable to get currentobject in your deployed environment. I can suggest using "remote debugging" to better understand the error.
I hope this helps.
The formatDate failed because you got an empty value for HolidayDate.
My suggestion is, in the first block, filter out records that have HolidayDate = empty and it should work.