admin/whos_online.php error: Maximum execution time of 60 seconds exceeded

[lumpylumpy]

Hi, i have zen cart1.38a, have a few addons on it, everything working ok, (have made some modifications prior to this problem, so first thing to try was to revert to backups for the files modified)
Yesterday i noticed that the admin/whos_online.php is taking a long time to load, around 1 minute. Also all visitors have active carts icon(green) assigned to then, even thou there shopping cart is empty and customers ho have items in the shopping cart , these dont show up in admin/whos_online.php. I have also installed User Traking mod and there active customers with shopping cart are assigned User Shopping Cart :session expired, so nowhere is the shopping cart off customers displayed in the admin.
Also noticed that i get an error in the logs like this:
PHP Fatal error: Maximum execution time of 60 seconds exceeded in /home/xxxxxxxx/public_html/xxxxxxxx/admin/whos_online.php on line 416
or
PHP Fatal error: Maximum execution time of 60 seconds exceeded in /home/xxxxxxxx/public_html/xxxxxxxx/admin/whos_online.php on line 410
Around line 409-421 i have in admin/whos_online.php

for ($i=$start_cart; $i<$length; $i++) {
        if ($session_data[$i] == '{') {
          if (isset($tag)) {
            $tag++;
          } else {
            $tag = 1;
          }
        } elseif ($session_data[$i] == '}') {
          $tag--;
        } elseif ( (isset($tag)) && ($tag < 1) ) {
          break;
        }
      } 

I have tried to revert to full backups but no result, tried older backups also. I think that is has to do with sesions but dont know where to look for the problem. Did not trye older DB backup.
Any help is welcome.
Best regards Zoli.