It could be a conflict with another 3rd-party Excel addin and/or other software installed on the target system (even AV/IS software). Tracking the cause of such errors (Note: it's typical for vendors to disclaim any fault in their own products) can be very time-consuming. I note that the LockXLS change log (http://www.lockxls.com/changelog.asp) admits to past problems with some AV/IS packages, notably Norton, and USB protection, etc, so you should also make sure you're using the latest version. Note too that it's always possible an update for the OS and/or Office from MS might break the product - it all depends on how well written it is. Likewise an update for the AV/IS software or another 3rd-party addin might break the product...

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]