When you discard a Weaver it goes to your discard pile. If you then play it via its triggered ability, you get it out of your discard pile and put it into play, then do it's effect.
Some time ago a rule was changed, that now says that in order for a card to be played, it needs to be where the effect that plays it expects it to be (with some exceptions that aren't relevant here). The Weaver's Reaction expects it to be in the discard pile. If the discard pile was shuffled in the mean time, the Weaver left the discard pile, so the rule kicks in an prevents you from playing that Weaver.
In practical terms, the card is really "lost", it's at an unknown location inside your deck now.