Production Web App Bug Fix

Our live web application occasionally blocks users from completing core actions like signing in and submitting forms. Each incident hurts conversion and support volume, so I need an experienced engineer who can step in quickly, trace the root cause, and ship a solid, production-ready fix... (Budget: $30 - $250 AUD, Jobs: Backend Development, Continuous Integration, Debugging, Linux, MySQL, PHP, Software Testing, Testing / QA, Web Development)