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I dislike any error comments that include the word OR. When they found the error they knew the cause, and they knew where the error was. Why not tell me? They do not because they are lazy.
If it takes a long time to load your file, then determine the time to where the failure occurs. This will give you an estimate of where to look in the program.
Otherwise or in addition to the above do the following:
Eliminate the second half of the program and test for the error. Continue this type of binary breakdown to isolate the region of the program where the error occurs. Whether you get the error or not determines where you look next.
Suppose no error in the first half of the program, go back to the original program and eliminate the last quarter, and test again. etc.