Hi Kev, thanks for the help onthis.
Line 3501 in class.orm.php is in this function

    function _cast($record) {
        $i=0;
        foreach ($record as &$f) {
            switch ($this->types[$i++]) {
            case MYSQLI_TYPE_DECIMAL:
            case MYSQLI_TYPE_NEWDECIMAL:
            case MYSQLI_TYPE_LONGLONG:
            case MYSQLI_TYPE_FLOAT:
            case MYSQLI_TYPE_DOUBLE:
                $f = isset($f) ? (double) $f : $f;
                break;

            case MYSQLI_TYPE_BIT:
            case MYSQLI_TYPE_TINY:
            case MYSQLI_TYPE_SHORT:
            case MYSQLI_TYPE_LONG:
            case MYSQLI_TYPE_INT24:
                $f = isset($f) ? (int) $f : $f;
                break;

            default:
                // No change (leave as string)
            }
        }
        unset($f);
        return $record;
    }

specifically the foreach line.
This function doesn't appear to have changed since 1.18.1 but many other functions in this file have changed, at least in the way they are declared.

I've enabled the two settings in ./bootstrap.php and it now echos the error to the screen as well as a longer one into the apache log files
On screen it says
Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 8192 bytes) in /var/www/help/include/class.orm.php on line 3501
in the log file it says
[Tue Feb 18 10:50:25.675654 2025] [proxy_fcgi:error] [pid 582218] [client 172.16.2.23:45948] AH01071: Got error 'PHP message: PHP Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 8192 bytes) in /var/www/help/include/class.orm.php on line 3501', referer: https://<my_url>/scp/index.php

This is an instance we have been using for some time. It has quite a lot of tickets. I've tried viewing smaller ticket lists (like assigned to me etc) to see if that helps but it still runs out of memory, even on shorter lists.

further update as of 2025/02/18 11:44 a clean install on the same server with a new database works fine. I swapped in the old database by changing the lines in the ost-config.php file and the same issue occurs. So it seems to be related to the database, rather than the server software or the files copied in the update

further update as of 2025/02/18 17:31 : original instance has been restored to the backup and is working fine on 1.18.1. The backup has also been used to restore to a new domain with its own DB restored from the backup as well. The DB config table needed a few search and replaces to correct the settings to point to the new domain name.
Copied the updated files across, restored the ost-config again and logged in to the admin interface.

Still get no tickets showing but the error is different this time. [proxy_fcgi:error] [pid 629011] (70007)The timeout specified has expired: [client 172.16.2.23:55088] AH01075: Error dispatching request to : (polling), referer: <URL here>

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