Had a non eventful Moodle 3.6.1 upgrade to 3.6.4 yesterday and after the upgrades on plugins and themes cleared cash tested activities in classes all we well with full admin functions. Tested on Firefox and Chrome.
This morning logged in and lost use of Site Administration > admin/search.php
error "Coding error detected, it must be fixed by a programmer: Failed to unserialise data from file. Either failed to read, or failed to write.
I can still edit courses but that is about it.
Thanks fore any help.