I am unfamiliar with the "worksheets" object... lets check the help...

"For an Application object, returns a Sheets collection that
represents all the worksheets in the active workbook"

Could the "active workbook" be changing?

Try accessing the workbook and worksheet explicitly via objects,
stored at the start before anything can change. Then work out where it
is changing, if it is.

Dim oBook As Workbook
Dim oSheet As Worksheet

Set oBook = ActiveWorkbook
Set oSheet = oBook.Sheets("User ID Control List")

Phil.