I just ran into something new. I was going to enable the Antibot module on an existing site that has been working fine for the last year. When I clicked on the Save Configuration button, I got the following message:
The entity_token module is missing, so the following modules will be disabled: rules_admin, rules.
Would you like to continue with the above?
Any ideas why I would be getting this message on an existing stable site. I suppose the easy thing to do would be to simply add the entity token module, but I'm curious about what is going on here? If Entity Token is required, how come I'm only getting this message now.
Is this related to recent module or core updates?
I agree with @oadaeh - this probably happened when Rules was upgraded without using the UI, or perhaps someone removed the file accidentally.
As a side note: Drupal (a few years ago) incorporated a check for missing module files and it currently shows a persistent message in all pages:
That never made it into Backdrop. The only time backdrop checks for missing files is when you open the module installer form. Perhaps that would be a good addition? I've been bitten by this a few times in the past. If your site doesn't need any of the missing file functions, you won't get any errors and the site will be functioning normally, even with the missing ones, which seems to be the case with yours.