MentionClient not found #3363

While trying to do this:

Create a Status update

I encountered this error:

Class "IndieWeb\MentionClient" not found

Some other notes:

Despite that, the post was created

Give us some context:
I just updated to version = '1.6.4'
build = 2026022801

I have not enabled StandardSiteSync.

• Any warnings or errors in your admin/diagnostics page?
  Cannot see the diagnostics page with 2026 Theme! No problems reported when I run it from Solo theme.