[Radek]

That's bad. Perhaps, hacking gurus there will be able to cheat the invitation back but I am not a hacking guru.

Anyway, you have two theoretical possibilities of recovering the invitation "legally".

(1) The invitation never vanishes. Therefore, cast Spirit Eye and investigate all "white dots" on your map. That means, do not explore new areas and check only areas where you have already been with the invitation (including the dungeons). If the invitation is somewhere, then you should find it.

(2) If you have more invitations, then try to steal the invitation from the guard after giving it to him and use it again with another quest owning character. I've never tried it so that I don't know, whether it is possible.

As far as vanishing quest items are concerned in general, do not forget two things:

(1) Stealing creatures (all ratlings, raiders, higher level trolls and several others) can steal everything unequipped, including quest items or such items like the Signet Ring or even the Mavin Sword. When they die they always drop the stolen items (except money!) so that you can get them back. Always check what was dropped by stealing creatures! If you have special items in your inventory (keys, quest items, etc.) then always check your inventory after a fight against stealing creatures, whether the special items are still there!

(2) The town bug: If you pass items from one character to another in a town or in a town building (inn, pub, guild, etc.) then always open the new owner's inventory! Due to a bug in the game, the changes made in the inventory in a town aren't "stable" until the inventory isn't diplayed. Therefore, the old owner will always "lose" the passed item (because his inventory is diplayed currently so that the change is stable) but the new owner need not be aware of getting the item until you display his inventory. If you put such character into the inn and get him back, then the item is lost.
Note: This cannot happen outside towns and town buildings. Passing items outside towns is safe.