Hi - just a note that we think we know what the issue is here and we're working on a fix that will be in the next update. It's related to using the mbstring php library, which is very common but not universal.
In the meantime, you should still be able to roll back your installation to remove the error and re-access your admin panel.
Follow the steps here:
http://www.nextgen-gallery.com/how-to-rollback-a-version/
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