Does Humility prevent Manifested creatures from being turned face-up?
You can still turn a Manifested card face up even if it would lose all abilities from a card like Humility.

This is actually called out explicitly in a ruling on Manifest cards:


  Unlike a face-down creature that was cast using the morph ability, a manifested creature may still be turned face up after it loses its abilities if it’s a creature card. (2014-11-24)


This is because the thing that lets you turn a Manifested card face-up is not an ability on the card itself that it would lose. Instead, the thing that lets you turn a Manifested card face-up is the Manifest rules themselves, which just say you can turn it face-up for its mana cost any time you have priority.