When a user uses the global search on the website, the search ends with the error
"Unable to acquire a lock for caching.". The user has a lot of courses (300+) in her profile.
Redis is used as caching server for application and sessions. These are the parameters in the config.php:

$CFG->session_redis_acquire_lock_timeout = 120;
$CFG->session_redis_lock_expire = 7200;