Line 613 of Classes/Integration/CoolUri.php is source of a warning if there is an array in the GET parameters.
E.g. if there is a param like "&my_extension_plugin[action]=show".
The problem can be solved by adding an additional check before encoding the value.
This is a possible solution (replacement for the line foreach ($params as $k => $v) $params[$k] = $k . '=' . rawurlencode($v);):
foreach ($params as $param => $value) {
if (!is_array($value)) {
$params[$param] = $param . '=' . rawurlencode($value);
} else {
foreach ($value as $subParam => $subValue) {
$params[$param] = $param . '[' . $subParam . ']=' . rawurlencode($subValue);
}
}
}