Valgavoth, Terror Eater does not let you cast "Target Opponent" spells in 1v1.

Cards exiled with [[Valgavoth, Terror Eater]] are uncastable in 1v1 if they want to target an Opponent. Ex. [[Duress]].
It seems the TargetOpponent filter assumes the owner to be the controller and therefore not being a valid target and thus the spell fails to cast.
The spell is highlighted as castable but trying to do so result in no effect.

Does not seem to be a problem in multiplayer. (Only tested against AI)
It let's you target any opponent including the owner.

Reproduce the issue in a 1v1 AI game using Valgavoth, Terror Eater and Duress, then trace the TargetOpponent filter used when casting exiled spells. Compare target validation in 1v1 and multiplayer; done means an opponent-targeting spell can be cast from exile against the opponent without changing its multiplayer behavior.

Written by the indexing model from the issue text.