First of all, are there any errors in the Console? Just scroll it through quickly. If the server is Online even if there are few errors, it should be fine. If there are any specific errors, check out which mods/plugins are causing this and let the client know.
Go check the Game Panel -> Properties tab, find “VIEW-DISTANCE” and check the value of it. The default value is 10 (chunks), if the value has been increased to over 20 chunks, that’s probably the cause of the error. Let the client know to decrease the value.
When the client has very bad internet connection, even the value of 10 will not let them load out from the bug. So it’s safe to decrease the value to 2-3! Then let them join, and if everything works - change the view-distance back to the default value (and restart server again).
