Hello

by typing in the key to the plugin, it displays "ERROR: API Key invalid! Please fix it and toggle plug-in."

Try to unclaim the server then re-add it. 

it did not help !?

I got the answer from 4Netplayers:

please check the api-key.
The plugin is telling you that the key is incorrect.
If you think that you are using the correct key, please contact bf4db.com

You can send me a PM with your eMail and I will send you version 2.0.10 from our plugin! Maybe that will help...