Ferret 'File Not Found' error with DRB

We’ve installed Ferret and DRB Server for the first time, for Rails,
along with acts_as_ferret, and for a couple of weeks in development we
got no errors, but now we’re starting to see an error like this:

File Not Found Error occured:
tried to open “/[app_name]/[model]_8e_0.del” but it doesn’t exist:

The error appears random – e.g. we can do a POST to update a document
and it will fail, then (Reload and) fail again, and then on the third
try with the exact same operation, it works.

One poster says this error occurs when one is NOT using DRB server. (So
this makes me wonder – perhaps somehow DRB is getting bypassed – like
we think DRB is doing what it’s supposed to but we’ve overlooked a
critical config line?) Or has anyone got any other explanation for