Autoloader problem after upgrade to version 4.6.8
This error can occur when there is a clash with a Composer autoloader for another module, e.g. the Currency module.

A README has been added to currency 7.x-2.5 re this error, advising: 'try manually updating Currency's Composer dependencies by running the command "composer update" in Currency's root directory.' This worked for me. Vandalf isn't using Currency but maybe a similar issue with another module? Try searching for files called composer.json under your modules directory.