Fix Magento Post-Migration Error

I’ve just moved my Magento store to a brand-new server that’s running PHP 8.4.13. Since the switch the web domain throw the generic message:

“An error has happened during application run. See exception log for details. Could not write error message to log. Please use developer mode to see the message.”

When I force the old server to use any PHP 8.x build I hit a 503, so I suspect a compatibility issue. I haven’t confirmed whether every module is PHP 8-ready,

What I need from you
• Diagnose the root cause on the new server (SSH, logs, developer mode, etc.).
• Update, patch or suggest replacements for any non-compatible extensions or code.
• Adjust PHP and Magento settings so the site loads cleanly under PHP 8.4.13
• Leave the store fully functional—frontend pages, checkout, and admin—with no 503s or fatal errors.
• Provide a short summary of the fixes you applied so I can replicate them on staging.

Magento CLI, composer, and full server access are available the moment we start. If you’ve handled similar PHP-version jumps and extension conflicts before, this should be straightforward. Let’s get the shop back online.