Power pivot issue - invalid numeric representation of a date was encountered

[AuroraBorealis13]

Hi, I inherited a power pivot worksheet which all of a sudden started misbehaving. it has several measure custom formulas like the one below.
There is a formula corresponding to each col, differing in the highlighted portion.
all of a sudden, refreshing started giving the above error, complaining about the 2Y.

I checked the valuedate_list table, which is laid out like so:

ValueDate	col_1M	col_YTD	col_3M	col_6M	col_12M	col_2Y
Jan-18​	1/31/2018 0:00​	1/31/2018 0:00​	11/30/2017 0:00​	8/31/2017 0:00​	2/28/2017 0:00​	2/29/2016 0:00​
Feb-18​	2/28/2018 0:00​	1/31/2018 0:00​	12/31/2017 0:00​	9/30/2017 0:00​	3/31/2017 0:00​	3/31/2016 0:00​

and all the dates are in proper date format.

I did find threads re this error but only as it applies to BI, so any suggestions/ ideas really appreciated!

thanks

2Y:=VAR Calculation_EndDate = MAX(ValueDate_List[ValueDate])

VAR Calculation_StartDate = MIN(ValueDate_List[col_2Y])

VAR Dates = DATESBETWEEN(Facts[ValueDate],Calculation_StartDate,Calculation_EndDate)

VAR ProductFn = CALCULATE(PRODUCTX(Dates,1+ Round([Return Internal],4))-1,Dates)

RETURN

ProductFn