The syntax of the SQL command was wrong - the Group By clause should be last. The part after that belongs to the Where clause. Also, you need to specify an aggregate function in you select if you are using Group By.

In the activate.php script, I changed the line

$groupquery = "SELECT g.gid,g.group_name,count(q.qid) as count from {$dbprefix}questions as q RIGHT JOIN {$dbprefix}groups as g ON q.gid=g.gid WHERE g.sid=$postsid group by g.gid AND g.language='$baselang' AND q.language='$baselang';";

to

$groupquery = "SELECT min(g.gid),min(g.group_name),count(q.qid) as count from {$dbprefix}questions as q RIGHT JOIN {$dbprefix}groups as g ON q.gid=g.gid WHERE g.sid=$postsid AND g.language='$baselang' AND q.language='$baselang' group by g.gid ;";

This worked and I could activate the survey.