in include/class.ostsession.php
Just add this line:
$this->data->session_data = "";
After:
catch (DoesNotExist $e) {
$this->data = new SessionData();
So it will be:
catch (DoesNotExist $e) {
$this->data = new SessionData();
$this->data->session_data = "";
Then try to login again make sure cache is clear