The site is not valid. The 'Pages' document library is missing.

ULS Log tells me that:

The site is not valid. The 'Pages' document library is missing. ......

Solution to this problem was really simple.

I've found a blogentry on blogs.technet.com, which tells me to update the "PAGES ID", and it worked well.

$web = get-spweb "https://site-collection/path-to-affected-site"

$correctId = $web.Lists["Pages"].ID

$web.AllProperties["__PagesListId"] = $correctId.ToString()

$web.Update()

$web.AllProperties["__PublishingFeatureActivated"] = "True"

$web.Update()

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