I'm guessing this is a function call and it's getting a text value rather than a date. Assuming you're building this as sql in vba, suggest trying
Code:
"UPDATE POInstall LEFT JOIN POSurvey ON POInstall.POID=POSurvey.POID
SET POInstall.NewEQApDate = dateAddWeekday(#" & [Forms]![POSurvey]![TSRApCoDate] & "#,1)
WHERE ([Forms]![POSurvey]![TSRApCoDate]) Is Not Null And POInstall.NewEQApDate Is Null;"
I leave it to you to add the line continuation characters or otherwise deal with the line breaks I've got that vba won't like.
Alternatively, you could test the sql as
Code:
...SET POInstall.NewEQApDate = dateAddWeekday(#12/07/2017#,1)...
and also
Code:
...SET POInstall.NewEQApDate = dateAddWeekday(12/07/2017,1)...
I'd expect the latter to fail, which ought to replicate the situation you have by referencing the value of a textbox, which usually looks like a string to sql and not a valid date.