Are you trying to put a string in a number field with the NZ replacement?

No.  It fails every instance of Nz.  I created new queries to test the problem and even the most simple calculated query field would generate the error.  I could open a separate file and create a calculated query field with Nz that works perfectly.

Does it do it in a fresh query with no fields being used in it, just do an expression like

Test: NZ(DLOOKUP("CustomerID","CustomerT","ID=1"),0)

Is this a top level query? Or does it have other queries in it? Sometimes the problem can be in another query that you're bringing in and then you have more to look through, can you upload a screenshot of the query/related queries if any?

Tim -
If this was working and then it just started not working it may be related to this Bug Alert

They pushed a fix supposedly yesterday.  You may have to manually update your Office 365.

I resolved the problem but I am still unsure of the true cause.  
Here's what I did:
1) I opened an Immediate window (Ctrl+G) and typed: ? Nz(Null, 0)
2) The window returned 0 so the problem seemed to be somewhere else than the calculation of Nz
3) Since the message box says a VBA module contains a syntax error I deleted the only module in the file (It was for a custom function)
4) Queries with Nz work exactly as expected now
5) My only problem is that I do not understand what was wrong with the VBA for the custom function (VBA below).  

Option Compare Database

Function TotalSec(DurHr As Variant, DurMin As Variant, DurSec As Variant) As Variant

If IsNull(DurHr) = True And IsNull(DurMin) = True And IsNull(DurSec) = True Then

TotalSec = Null

Exit Function

Else

If IsNull(DurHr) = True Then DurHr = 0

If IsNull(DurMin) = True Then DurMin = 0

If IsNull(DurSec) = True Then DurSec = 0

TotalSec = DurHr * 3600 + DurMin * 60 + DurSec

End If

End Function

Tim, a few notes... I HATE Variants. Avoid them if you can. I also recommend not setting values equal to Null. Make it 0 if possible. Strings empty "". Null is a pain in the ass, pardon my French. It looks to me like you should be able to use Longs for your function instead of Variants. If you want to allow the user to not pass a value, just say something like:

Function TotalSec(Optional H as Long = 0, Optional M as Long = 0, Optional S as Long = 0) As Long
  TotalSec = (H * 3600) + (M * 60) + S
End If

Problem solved. No Variants or Nulls needed.