I cannot add ip adressess anymore

After updating to 4.2.3 and later to 4.2.4, your normally brilliantly functioning plugin refuses to edit or add any records. Is that a known error?
A second question: I have one school in the list of IP adressess that is not functioning properly. I checked there IP- adress myself and also with the aid of the internet service provider: it is the correct ip-adress. I haved used the “admin IP plugin” as well to check the ip-adress. No proxy is being used. Do you have any clue where to find the answer.

https://wordpress.org/plugins/ip-based-login/