I am getting a similar error, I removed the 1 ticket filter in the system and no change, I increased the memory_limit ti 512M and the error is still coming up. I have been using this same version and configuration for several months with no issue. This error first popped up a few days ago and now it is running down slow.
/include/class.osticket.php on line 263
[22-Apr-2019 16:08:56 UTC] PHP Fatal error: Allowed memory size of 536870912 bytes exhausted
About this osTicket Installation
Server Information
osTicket Version v1.9.6 (9adad36)
Web Server Software Apache
MySQL Version 5.6.43
PHP Version 5.6.40
PHP Extensions
gdlib Used for image manipulation and PDF printing
imap Used for email fetching
xml XML API
xml-dom Used for HTML email processing
json Improves performance creating and processing JSON
mbstring Highly recommended for non western european language content
phar Highly recommended for plugins and language packs
fileinfo Used to detect file types for uploads
PHP Settings
cgi.fix_pathinfo "1" is recommended if AJAX is not working
date.timezone UTC
Database Information and Usage
Schema pronlinedb (localhost)
Schema Signature b26f29a6bb5dbb3510b057632182d138
Space Used 174.28 MiB
Space for Attachments 0.26 MiB