phplist throws "curl dependency error" error on submitting registration form

Upgraded my phplist installation from v2 to v3 and put it on a new server. All seems to run perfektly but then I heared, that the registration has a problem. I tried it and I got the error too:

curl dependency error

simple text-message, no more information. I found completely nothing about this as the codeline where it is programmed to throw the error. It tests if the function "curl_init" is availible. So I installed the php5-Library "php5-curl":

apt-get install php5-curl

After a restart of apache

service apache2 restart

it worked!