#ref! & sum

I want to sum a column that pulls its information from another worksheet. Other columns then reference these cells for calculations.

Numbers pulled from the other worksheet are mostly pulled from time differences which are then converted to decimals (using the INT function). All calculated cells seem to be returning valid results and all cell references (both inter & intraworksheet) are correct. However, cells that have a SUM in them =SUM(E5:E41) return #REF! with note "Moving or deleting cells caused an invalid cell reference, or function is returning reference error" (2007). Have tried all different formats in both origination and destination cells.

Simple solution I'm sure, thank you for your help,
Ray