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The situation that you're defining here could be a file permission issue. The data itself could be not writeable. The error that you're getting could be a vague message, in other words, wrong message. Outlook tends to send out wrong messages on various occasions.
The reason I say it could be a permission issue because end user can't edit them, certainly it's not a memory issue so it'd be safe to rule that out, this then comes to forged message. You'll need to look in other directions and first one would be permission.
The issue of moving these names, can't be memory on that one either since it's only 300 names. That would take no more than 4GB of available memory + the page file. So in that case you'll need to look at other directions too.
This could also be the reason why you couldn't find any information on google and MS.
Keep me updated and ask any questions you may have and I'll be sure to help out as much as I can.