Can't join that player

So im fairly new to PSO2NG but I play alot of games and have a good understanding of how things work but for the life of me i cant figger this out.

Ok so im able to join other people's parties and invite them to mine but i cant join both of my sons parties, so we were trying out if it was a IP issue by joining them over my console and it worked, I however cannot and get text saying i cant join that player! Screenshot_20210802-231730.jpg !

Out of curiosity did you (or they) accidentally block one another via Xbox LIVE or on the in-game friend menu? If that is the case, it might be why you might not be able to join each other. Unfortunately, I am not aware of any other solution or causes for this particular issue.