Can't Add Block to Area - "Invalid Area"

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I've got a site where some areas on some pages give me a pop-up that just says "Invalid Area" when I try to add a block to them.

Similar issue seemed to happen here:https://github.com/concrete5/concrete5/issues/852...
... but apparently only happened on HR blocks and looks like its 5.7 anyways.

My issue is on 5.6.3.3.

Any help would be greatly appreciated!

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