Incompatibility between VASSAL versions in FAB The BULGE

I was playing the campaign game in FAB the BULGE using Vassal 3.6 (I do not remember which version exactly, I assume tha last one before 3.7). I was the Allied player and my opponent the German. When we switched to 3.7.0, I could open the old files without a problem being able to view only the Allied units, however my opponent received a prompt to choose either observer or solo. Could not select German. I did not receive such a prompt. He eventually opened the last file under “solo”, saved it an emailed it back to me. I still could not view the German units, nor did I receive any prompts when opening the file. Is there a solution to this?

Almost certainly, your opponent changed his personal Vassal password at the same time as he installed Vassal 3,7. The side taken by a player is connected to the password recorded in the Personal tab of the preferences. He will need to change that password back to whatever it was before.