When a police officer arrests someone based on an improperly issued warrant the warrant protects the officer from liability?

If the officer reasonably believes the warrant is valid, then the officer is protected from any legal (or civil) consequence. However, the key phrase to that is "the officer reasonably believes".

If someone manages to prove that the officer "should have known better", or that the officer lied in order to have the warrant issued, the officer can be held liable, as they knew the warrant was invalid.