I don't know if there are fixes in that area. It is best to talk to GPSoft directly about that. It's likely that the issue is tied to something unique about the way your NAS device works so you might be asked to run a debug version of Opus or use some monitoring tools to see what's going on if GPSoft can't reproduce the problem on the hardware they have available. (If you're lucky the NAS manufacturer might loan GPSoft a device to try to repro the problem with, but that's never guaranteed.)
Check your NAS has the latest firmware/drivers, too. It could be a bug on its side that has since been fixed.
It might be worth having a look at what Process Monitor reports Opus (and other programs) doing when you delete directories from the NAS. That might reveal what Opus does differently and where the operation fails.
I feel the need to rant. None of the quote below will help solve your problem, which I hope can be fixed, but it might make it easier to understand why there's a problem which only seems to affect Opus.
[quote="Nudel Rant"]It seems that NAS devices vary greatly in how they handle less commonly used features, something which has tripped up the Windows developers in one example (which has meant we'll be stuck forever with the slow method of reading directories):
blogs.msdn.com/oldnewthing/archi ... 73160.aspx
blogs.msdn.com/oldnewthing/archi ... 64809.aspx
That problem turns out to affect many NAS devices from different vendors. I've experienced a bunch of other problems, too.
The Samba-based NAS devices are usually lax at sending change notifications because they detect changes by polling, sometimes every few minutes(!), if at all (!!). (I gather this is improving in newer Sambas, if you use the right Unix/Linux filesystem and the right patches and...)
Some NAS devices don't support Unicode (including some Samba-based ones, if they use the wrong build/version of Samba).
The LinkSys NSLU2 that I bought had all of these problems and also had such a cripplingly slow processor that it couldn't read/write data anywhere near as fast as the network and USB2 drives allowed... (That thing is now sitting in a drawer with other electronic junk. Could make a nice embedded Linux web server, but next to useless at the job it was sold to do...)
I'm sure there are some good NAS devices (I know Jon has used one that he said worked well, though I don't know the make/model) but it seems there are a lot of bad ones, too. I decided it was easier to build low-powered Windows boxes to get SMB network drives, rather than find a good embedded solution!
None of which helps much, but it's good to rant. 
[/quote]
(btw Opus never appears in tests and german special-magazines, only TC & Co. 
)