I ran into a similar error with as useless an error message. It could be a permissions problem. I've found that often times when you get the "Unable to complete this operation" message, it's because you don't have permissions to do what you're trying to do. I would check what permissions your application is using and possibly use elevated permissions for that operation instead. Or maybe open up the permissions on the archive giving you problems.

I ran into a similar error with as useless an error message. It could be a permissions problem. I've found that often times when you get the "Unable to complete this operation" message, it's because you don't have permissions to do what you're trying to do. I would check what permissions your application is using and possibly use elevated permissions for that operation instead. Or maybe open up the permissions on the archive giving you problems.

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I've also recently seen a lot of strange permissions behavior regarding inheriting permissions from a parent site. You might try dis-inheriting permissions (or re-inheriting depending on your current setting) and see if that makes a difference.