I have applied a temporary fix for the participants token import/editing error.
In the file application/models/Token.php, I commented out lines 318 and 319 and added an entry.
My code now looks like this:

    // According to Yii doc : http://www.yiiframework.com/doc/api/1.1/CSecurityManager#generateRandomString-detail

$pattern = Yii::app()->getConfig("allowedcharacters_pattern_token");

return preg_replace($pattern, '', $token);

    return preg_replace('/[^A-Za-z0-9_]/', '', (string)$token);

I am awaiting the official fix.