HINT: It affects ALL MC Versions Multiplayer since it comes from Your Session Servers!

The Summary contains all and no, I am not going to search for it! I go to You to tell You
that single sessioning is a rly, rly bad idea!

We have bougth the game for at last 19.99 € and aren't allowed to join multiple servers at the same time as long as we can manage the load ech client will give "eat"?

This is a point which should be fixed at your side rly fast and yes this is a bug, definitly.
Other games are also allowing multi sessioning so here it shouldn't be the problem.

Cheers