Field UI Local tasks are incorrect on any entity

the problem is in object to array conversion in those functions:
entity_get_form_modes()
entity_get_view_modes()

$form_modes[$form_mode_entity_type][$form_mode_name] = (array) $form_mode;

$view_modes[$view_mode_entity_type][$view_mode_name] = (array) $view_mode;

i think it's critical issue after #2111823: Convert field_ui / Entity local tasks to YAML definitions is resolved

There's no problem with entity_get_form_modes() or entity_get_view_modes() here.

The problem is in \Drupal\field_ui\Access\FormModeAccessCheck and \Drupal\field_ui\Access\ViewModeAccessCheck

When on a path like admin/structure/types/manage/article/display, it uses the route from \Drupal\field_ui\Routing\RouteSubscriber, which has the 'entity_type' added to the $defaults. But when on a module defined route like admin/structure/types/manage/article, it does not have the entity type specified, and those access checks fail immediately.

@tim.plunkett
it fails on any path after the cache flushing
also, it sets a wrong cache value to "entity_view_mode_info:en"

a:5:{s:7:"comment";a:1:{s:4:"full";a:12:{s:2:"id";s:12:"comment.full";s:4:"uuid";s:36:"06ab5b16-e197-4242-b72c-4453793fabba";s:5:"label";s:12:"Full comment";s:16:"targetEntityType";s:7:"comment";s:6:"status";b:0;s:5:"cache";b:1;s:13:"

that value wouldn't be unserialized next time

ahhh, yep, understood...
please ignore my #7 comment

This fixes it for content types, accounts and taxonomy vocabularies.

That's one way to fix that, but how would you know to do that? Only Field UI needs it.

Right, different approach, works fine too.

That is *much* better! Now we just need tests.

The test and the fix looks good, sorry for breaking this :)

This fix makes totally sense.

Nice catch!

Committed and pushed to 8.x. Thanks!