Wrong variable in if condition when showing message of invalid token (application/controllers/survey/index.php):

[ ... ]
elseif(strtotime($now) < strtotime($oToken->validfrom))
{
$sError = gT("This invitation is not valid yet.");
}
elseif(strtotime($now)>strtotime($oToken->validfrom))
{
$sError = gT("This invitation is not valid anymore.");
}
[ ... ]

In the second "ifelse" condition variable should be "validuntil"