problems installing the dashboard

I have been trying to install the dashboard since last week and continue to get the error message:

"Your server had a problem connecting to WPMU DEV: "GnuTLS recv error (-9): A TLS packet with unexpected length was received. Please try again."

My service provider assures me that curl is activated and sent me the following answer:

"Sorry but that error comes from the php code not from the server. You'd have to check with the programmer of the code why it isn't working."

I have tried to solve the problem by adding the following code to the wp-config.php file:

define( 'WPMUDEV_CUSTOM_API_SERVER', 'https://premium.wpmudev.org/' );

Unfortunately this doesn't work!

Please help me to connect the dashboard

Thanks

Ken