Everything hinges on this claim.
If Cohen didn't visit Prague, the Steele dossier and pretty much the entire collusion case is obliterated.
On the other hand... If Cohen did visit Prague in 2016... Well... Let's just hope he didn't.
He behaves as a man that can prove he didn't. I'll bet they have absolutely no evidence that he did, and therefore have relied on a claim by some unnamed source which will also not be able to prove it.
If they can't prove it, and his passport shows no evidence of it, it just becomes hearsay, and this demolishes the foundation upon which their warrant was based.