I have used this wallet sever times before to send or receive. I closed, downloaded and tried again to send to multiple addresses. Error message: https://i.stack.imgur.com/RK47s.jpg
The dust threshold depends on the min relay fee, which is a server setting. Thus, it is server dependent. At this point, this threshold is not sent by the Electrum server to the client, but it is a default value, hardcoded in the client. That default value might be lower than the value set by the server you are using. Try to use another server.