Data Validation List Formula causing Error - List source must be a delimited list

I am trying to used nested if statements with AND and OR functions within a data validation list and am getting the following error:

"List source must be a delimited list,or reference to a single row or column"

I had the formula working previously but am unsure of what is causing the error.

Formula:

=IF(OR(ProgramCode="GMT561",ProgramCode="GMT967",ProgramCode="GMT968"),SOD_SW_Version,IF(AND(RST="ESS",MYC="2014"),K2xx_SW_Version,IF(RST="SBZA",SBZA_SW_Version,IF(AND(RST="ESS",MYC="2013"),ESS_SW_Version,""))))

ProgramCode, MYC and RST are names for cells on the main sheet of the workbook.
SOD_SW_Version, ESS_SW_Version, SBZA_SW_Version, and K2xx_SW_Version are names of lists in columns on another sheet within the workbook.

All names are global in that their scope is the entire workbook not just a single sheet.

The purpose of this forumula is to populate the validation list with one of the named colums based on the values in either MYC, RST, and ProgramCode or a combination of the 3 with the OR and AND functions.

Can anyone help me to figure out what is causing the error?

Formulas like that will give an error when first entered unless you have set the other cell values such that this formula resolves to one of the named ranges in the formula, at least temporarily. Do that, enter values in the other cells, then enter you DV formula, then you can clear the other cells and start using it.