Call to undefined function json_encode() when trying to login / register

Hello,

I just set up emoncmsv7 with redis and timestore. The database and everything went fine.

But when I tried to register a user, an empty red error-box shows up und nothing else happens.

So I looked at the firebug console and there I got the following error:

<br />
<b>Fatal error</b>:  Call to undefined function json_encode() in <b>/var/www/emoncms/index.php</b> on line <b>126</b><br />

 

I guess it is a very simple issue, since i already got the emoncmsv7 installed on another server. But now something went wrong...