Mail merge data list not showing up

Hi, Hopefully I can explain the issue ok. I am using a mail merge document that has worked fine before and with using the same excel data source but now the data is not showing in the recipient list. The process I have used is:

When opening the Word document, the initial command box still has the correct source file and so I say 'Yes' to continue - all seems to be fine as the document then opens ok.
Use Step-by-step mail merge wizard function.
ISSUE: on Edit recipient list - the box is blank, it is not listing the contents of the Excel data source, even though the smaller box below called 'Data Source' has the correct named data source file.
When this first happened, I checked Options/Advanced/General - Confirm file format conversion on open - it was ticked
I then unticked this option and tried again - this time it did bring up the data and I was able to Edit recipient list by selecting one row but when I selected OK, it just went back to the blue circle of death and it just gets stuck, I have to close/restart Word.

This mail merge document has worked fine before but I recently have edited it a lot but only by moving data to other sheets.
Is it likely that my document has just become corrupted somehow and if so is there anything I can do about it please?

FYI, previously when selecting the data source it only listed OLE DB as the format to be selected which has worked fine.
I use Professional 2019

Many thanks in advance.

I have just remembered, I had added some simple Date formulas (=DATE(YEAR(ref),MONTH(ref)+3,DAY(ref))and some conditional formatting to the sheet in question so don't know if that has anything to do with the issue?!