After uploading the 4.7.21 plugin the CiviCRM admin pages loaded ok with the status at bottom indicating new version and of course with a red 'System Status' warning. On clicking this to get to the status page to update db etc, got this error: Uncaught exception: You have requested a non-existent service "asset_builder".
Tried installing clean copy again. Same result. Resolved issue by manually entering the upgrade page url. This brought up expected option to upgrade db. All well after this. https://xxx/wp-admin/admin.php?page=CiviCRM&q=civicrm/upgrade&reset=1
Question: is this just our installation or have others had same behaviour, and did this fix resolve?