I guess you're using Meta Box Updater extension, aren't you? If that's the case, please remove it. You don't need it anymore, since the updater is bundled inside Meta Box.
If you are not, then please take screenshot of the License page after you submit the license key and post here. I just want to see the status message. It will show if a license is valid, expired or invalid.
I also just ran into this same warning after updating to 5.1.0 that my license key was expired. Deactivating Meta Box Updater solved the problem for me.
Anh - you can disregard the email I just sent in from your contact form about this same issue. I didn't see the instruction to remove Updater in your update email this morning. But I see it in the linked blog post.