[Evolution] Error sending mail --- no provider for protocol 'file'

[Paul Leyland <paul leyland vispa com>]

A few weeks ago my installation of Evo started generating an error when
sending mail --- for no obvious reason.  Evo itself wasn't changed so I
suspect that a third party is to blame.

After clicking Send, a yellow ! triangle appears, clicking on which
produces a message which states:

Failed to append to file:///home/pcl/evolution/local/Sent:
No provider available for protocol 'file'
Appending to local `Sent' folder instead.

This (appending to Sent folder) is precisely what it should do anyway!
Unfortunately, although it marks the sent mail for deletion it doesn't
actually delete it.  As a result, sent mails accumulate until they are
expunged manually.  Sometimes, they are sent out multiple times before
they are deleted ---but not always.

Any ideas what has gone wrong and how to fix it?

If it's relevant, its a 64-bit Gentoo system running Evo 2.26.3 and
everything is kept up to date on a roughly weekly schedule.

Paul