Credit Union changed account number, now new checks are being denied everywhere

Summary

Hello, my mother is in a deadlock trying to figure out why she can't use her checks. Her credit union changed her account number and issued new checks. They were denied when she would try to use them. After the problem continued, the credit union tried closing that account and opening a new one, and these checks are also being denied. The credit union claims this is the first time this has happened which I do find hard to believe. They aren't offering any other solutions and my mother&#3

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