When you try to see the preview of an object, the website freezes.
10 minutes after, you get the preview, with errors.

------------------------------------------------------------------------------
Error: processCache::_exclusiveLock() Apr 28 2008 12:07:48
Failed to execute SQL for function:

INSERT INTO ezdbfile (name, name_hash, expired, mtime) VALUES ('', 'd41d8cd98f00b204e9800998ecf8427e', 1, -1)
ON DUPLICATE KEY UPDATE mtime=-ABS(mtime), expired=1
Lock wait timeout exceeded; try restarting transaction
------------------------------------------------------------------------------
This is repeated 20 times.

Steps to reproduce

eZ Publish 4.0 is in a cluster mode.
The website (design and settings) is in an extension.
Edit an object and go on the preview.
The response time is huge, waiting for the mysql lock timeout.