Looks like the signature is valid but copying it from OP's post makes it look invalid. I went to the link OP posted above (his session), downloaded the Letter of Guarantee, checked again and it worked (verify here).

edit: Apparently there is an extra space after the first paragraph? (see).

Yes, you are right. You used the real website and the LoG is legit. Have you tried contacting them? Their email is support@privcoin.io (source).

If they don't answer to this, I will give them a negative trust and advise you to create a trust flag so I can support it.