Use cache keys which are compatible with supported backends (CiviCRM 5.4 vs Memcache)
Reported by @haystack in ~dev channel, suggested that it's a result of #174 (closed) changes.
After upgrading to CiviCRM 5.4 when using Memcache:
Memcached::get(/CiviCRM Session//_CRM_Activity_Form_ActivityFilter_851ce9ce57b6137a76ba5239cc2b6a6d_3667_container) failed: A BAD KEY WAS PROVIDED/CHARACTERS OUT OF RANGE
Memcached set (/CiviCRM Session//_CRM_Event_Form_Search_023fbd690622b4991938e06757805c3e_6468_container) failed
memcached/doc/protocol.txt has,
the key must not include control characters or whitespace.
Edited by xurizaemon