The data we see in logs is inconclusive. The server itself is running fine.
It looks a bit odd that you have three devices named Casier 02, 03, 04 that were all last seen by our server on 2021-08-05 16:53:49 i.e. that's when they last fetched a screen update. I don't know what to make of this. Does that by any chance ring a bell?
You could try to deleting those devices on espaper.com and re-registering them. However, I also noticed that they seem to be running a really old version of the device firmware (before we added the over-the-air update feature).