[imp] Invalid method "fetch" for actionID "download_attach"

[R Phillips]

IMP version 5.0.14, Horde Groupware Webmail Edition 4.0.3

I have been trying to debug multiple repeating instances of the
following message type:

servername.example.com HORDE: [ID 702911 local3.emerg] [imp] IMP_Imap:
Invalid method call "fetch". [pid 4807 on line 330 of "/path/to/horde/
imp/lib/Imap.php"]

This is generated at around line 80 of view.php in /imp as far as I
can tell, the line:

$contents = $injector->getInstance('IMP_Factory_Contents')-

>create(IMP::$thismailbox->getIndicesOb(IMP::$uid));

...and always for the actionid of "download_attach" in the "dynamic"
mode.  It doesn't always fail, only for some people some of the time.

Any ideas on how to start finding out the cause of this?  I have even
copied email that causes the issue and not been able to replicate the
issue for me and it doesn't seem specific to users or browser
versions.  I've seen a bit of double-encoding of the space character
in file name (eg %2520), but this doesn't seem to be the cause.

Where to look next?