I have searched a number of forums extensively for a definite fix to the issue of Juhani's side quest (Xor) messing up with the messengers on the game (so you wouldn't have any other one spawn after that). It was to no avail.
All I was able to find during my search was the usual defeatist answer "don't trigger Juhani's side quest until you have done everything else, it'll mess your game". That was most disheartening and irked me to no end, since I wanted a solution that didn't involve me giving up and starting up a new game (or finishing my current one without the messengers I hadn't triggered yet).
So I decided to take matters into my own hands and, needed be, bang my head against the wall until it fell apart. Guess what: it did.
So without further delay, here's how to fix your game so you can get the messengers you missed because of the stupid Juhani quest bug:
You'll need to get KSE to edit your save file. I recommend you save a different game just for messing with this (you shouldn't have problems during the procedure, but it is always better to be safe than sorry). After you get what you want you can undo the changes and go on with the game.
Open the savegame you want to fix with KSE. Under "Globals", you'll find the "Numerics" tree. Open it.
Search for the variable "K_XOR_AMBUSH". If you already completed Juhani's quest, it should read "2". Change it to "0".
This will have unlocked your messengers. Go into the game and get the ones you are missing. Save the game and quit.
Open your save game with KSE again and change the Xor ambush variable back to "2" (or whatever number it was). Your game will be restored in regards to your status on Juhani's quest.
Go back to your game and enjoy!
Note: I hope this will help the vast majority of people that got into the Juhani bug and missed things like Ziagrom and the Exchange special store. If your problem was the same as mine, it should work fine. If it doesn't, your problem may not have been directly related to Juhani's side quest.