Connection troubles

we have several Poppy Ergo Jr. here and while I had quite some fun with them already, nothing works anymore:
We connect the robot arm through the LAN port to a Mac and can open the poppy.local-website. I used to be able to play with the arm through snap!, but now we can’t get a connection. The monitor window only shows the broken red link, too.
I can’t find a troubleshooting guide for this problem other than some threads in this forum. I did deactivate the camera, even unplugged it on one of the robots, but that didn’t help, either.
What can we do?
Thanks in advance!

I can access individual motors through the Jupyter terminal and setting their id succeeds. But even that motor is then not visible to the page under http://poppy.local/monitor or to the Snap!-environment. The Snap! projects loads fine from the raspi…