Can you be sure that your opponent has also installed Potzblitz?
It's mandatory that both players have installed the mod (even the exact same version), else it will lead to crashes.
Unfortunately there's no way in the multiplayer lobby to mark challenges that were issued using Potzblitz. Potzblitz challenges will still appear as "standard" challenges for all players without the mod.
So maybe a player without the mod has accepted your challenge, leading to the crash. This match is now unplayable, contact Slitherine support to have them remove the match.
You also should delete all your remaining open challenges.
As said before if you want to play Potzblitz in multiplayer this can only be done if you can be sure that your opponent also has the mod installed.
It is therefore highly advised that players using Potzblitz issue private challenges using a password to prevent players without the mod unknowingly accepting such a challenge.
Use the Opponent-finder subforum to issue such challenges.
https://www.slitherine.com/forum/viewforum.php?f=219
If you are sure your opponent also had Potzblitz installed:
I need to have a look into the game's logfile, which is called
ctgw.log which is to be found in your Documents folder, so look in "My Documents\My Games\Commander The Great War".
NOTE: This log is overwritten every time you start the game. Please make a copy of the log after any crash/bug you want to report BEFORE reloading the game.
Put the ctgw.log file into a .zip or .rar archive and attach it to your post using the forum's "upload attachment" feature.
You can find the upload button in the bottom left corner of the reply box after you click on "full editor and preview".
Alternatively you can just copy/paste and post the last few lines of the "error" message at the end of the ctgw.log, usually this holds enough information to fix a bug.