Data Validation / VBA / Row Limit issue

Hi,

I am using the following macro to create and update a data validation list that contains dates. I am getting the following error message:
"Excel found unreadable content, Do you want to recover the contents of this workbook? If you trust the source of this workbook, click yes".Excel was able to open the file, by repairing or removing the unreadable content. Removed feature: Data Validation from worksheet 22 etc.

The code that I am using is:

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Is there a character limit for the data validation field or a row limit? I've tested it myself and do not get this error if the row limit (column B) which contains dates is 750 rows. It seems to be working without producing the error. If I add in extra data, where the row limit is now 1700, save the document and re-open, I get the error mentioned above.
I need allowance for about 3000 rows. The date field will hold one month of dates, and they can appear many times, but the data validation macro groups/filters the dates and places in B6 so they only appear once.

I cant tell what exactly this error message is trying to tell me.

I have read that I need use a string that specifies a range where the list is stored rather than using a string that contains the list of validation values separated by commas. Is this true? How do I change this so that I can attempt that? Do I put a named range in e.g."dates" , how do I incorporate it and remove the scripting dictionary if that has to be changed?

Many thanks.

I just typed the range in the formula1 section of the code and no error, but the validation box now doesn't appear with the dates in it for selection.