You flipped on Redis caching in the PrestaShop back office to squeeze out more performance, and now the storefront returns a blank white page or a raw HTTP 500. No admin, no front. Just a broken store and a nervous client.
This is one of the most common self-inflicted outages on PrestaShop 1.7 and 8.x. The good news: it's almost always a configuration mismatch, not corrupted data. Let's trace it.
Quick Diagnostic Cheat-Sheet
| Symptom | Root Cause | Immediate Diagnostic Command |
|---|---|---|
| Blank white screen / HTTP 500 on front and admin | PHP cannot reach Redis (wrong host/port/socket) | redis-cli ping |
| 500 only after enabling cache in BO | _PS_CACHE_ENABLED_ set with bad params in parameters.php | tail -f ~/logs/error_log |
Class 'Redis' not found in logs | phpredis extension missing for active PHP version | php -m | grep redis |
| Intermittent 500 under load | Redis maxmemory hit, evictions failing | redis-cli info memory |
| Cache never populates, slow store | Redis auth password mismatch | redis-cli -a 'pass' ping |
Step-by-Step Resolution
1. Confirm the actual error, not the browser message
A generic 500 tells you nothing. Enable debug mode temporarily or read the PHP error log directly. On cPanel or DirectAdmin, the log lives in your account root.
tail -n 50 ~/logs/error_logLook for lines like these:
PHP Fatal error: Uncaught RedisException: Connection refused in /home/user/public_html/src/Adapter/Cache/RedisClientFactory.php:74 Red redis extension is not loaded PHP Fatal error: Uncaught Error: Class