This has been quite tricky for us. I've been investigating this for months yet it continues to occur intermittently.
Two admins are active on the Magento backend. One admin begins experiencing err_connection_timed_out. The admin that most frequently experiences this is usually browsing customer or order data, however, when it occurs the frontend and backend throw the same error.
Our MySQL instance is using about 1/4 of the provisioned RAM and even less CPU %. I suspect that this could be a MySQL configuration issue, but am at a lost for what to check next.
Solved! Go to Solution.
I think you should enable developer mode on your site to checking it. To enable developer mode, you can do steps as below:
Open the file index.php and add the code below:
Mage::setIsDeveloperMode(true); ini_set('display_errors', 1);
Also, I think you should check table core_session. If you configure save session via database, you can try remove all data in table core_session to resolve the problem.
Hope this helpful!