It could be a lot of things, and with the very limited information that is provided here, it is hard to guess, as all we can do is assume. Sai did provide a good hint. The next step, once you ruled out the basics that I asked for (which we have to believe you are in place), you go step by step. Analyze what the difference is between the load that is working and the one that is not working. Make it simple, reduce it to a single row/column with one data cell only. Then write your string variables (and anything else for that matter) in the error log and check whether this is all as expected. It is similar to working on a huge jigsaw puzzle. Hard work and frustrating, but in the end diligence pays off and you will probably slap your forehead.