Moving messages does not work

[csc2ya]

I'm running a roundcube 0.9.4 installation on my server 2008 R2 web edition dedicated server. For the mailserver, i'm running hmail. I'm also running spamassassin for windows as a service using nssm as a wrapper for the service.

Every so often, moving messages to other folders by dragging them will not work. I don't get any errors, but nothing happens. The messages stay where they are.

I was previously running 0.9.3 and did a clean install of 0.9.4 today to see if that fixed the issue ,to no avail.

I have previously been able to fix the issue by restarting iis (iisreset in command prompt), but this now no longer seems to work. I've also tried restarting mysql, and hmail, but these also have not fixed the issue. I haven't yet tried rebooting the server (was last rebooted 2 days ago for windows updates), but I want to avoid this if possible as I don't really consider it to be a solution every time.

Edit: Have now tried a reboot, but this has made no difference.