By sprinkling "die()" statements around I've discovered the error is coming from inside the filemanager class. If I put a die() after allocating a filemanager object it's never reached, and instead the "Requested blog does not exist!" error is displayed.

I've now discovered inside _filemanager.class.php: $bloglist = $BlogCache->load_user_blogs( 'browse', $this->User->ID );

includes blog_ID=2 as its first item, which doesn't exist. I forget which one that was, but I figured it wasn't necessary to the site and so I deleted it. Apparently it's not deleted from everywhere if load_user_blogs() is finding it.

Now I guess I need to go into the database to discover why that blog id still exists there?

The problem was deleting the blogs through the admin screens did not delete rows in evo_bloggroups for the deleted blogs. Consequently, the SQL for getting the bloglist returned blog IDs that no longer existed.

Missing referential integrity?

I forgot to say, the SQL

delete from evo_bloggroups
where bloggroup_blog_id in (2,3)

seemed to do the trick.