after upgrade "non-existent" civi_flexmailer_required tokens

[Stoob]

Several times on several different installs since the 5.x series, after an upgrade, the following message appears where accessing screens in CiviCRM, both CiviMail and non-CiviMail related. The solution is simply to clear CiviCRM's cache, or civicrm/menu/rebuild. However I'm wondering why the cache clear that occurs during or after the upgrade itself is unsufficient and the cache must be cleared again

Sorry, due to an error, we are unable to fulfill your request...
You have requested a non-existent service
"civi_flexmailer_required_tokens".

[totten]

The issue is a good point. The same general phenomenon comes in up other cases (i.e. it's not particular to FlexMailer). There is an obscure option (CIVICRM_CONTAINER_CACHE=auto) which reduces the frequency of this error, and we changed the default in Civi v5.4+. There's more explanation in the PR:

civicrm/civicrm-core#12426

I haven't specifically re-tested Flexmailer upgrade-paths for this error; however, 12426 should make it a lot better, and I haven't heard new reports of this error since.

If the problem still happens after setting CIVICRM_CONTAINER_CACHE=auto, then please feel free to reopen as a core issue (with a link to this and 12426). I'd rather fix the general container behavior in core than put work-arounds into this particular extension.