I don't know how reproducible this is, but, it seems DOpus is missing something while entering credentials to remote machines. When I go to the remote machine, it asks me for credentials, but, even though I enter them right, it sometimes/often immediately opens the dialog again, like I've made a mistake. This is repeated for a few times and then DOpus stops trying and says the network path is inaccessible. However, if I go to the standard Windows Explorer, I go to the remote machine, enter credentials, it succeeds immediately. Now, if I return to the DOpus, I can browse my remote machine until the next reboot. I see this issue frequently on Windows 7 and 8. I don't remember I saw it on XP, but I might be wrong.
What type of server is at the other end?
Always Windows. Sometimes 2003, sometimes 2008R2. I'd say the problem happens equally on both, but I'm not 100% sure.
I'm not too proficient with networking, so I don't know whether this is important - my computer is on domain, but those computers aren't (they are just in a local network). But, as I said, Windows Explorer doesn't seem to have problems accessing them.
If you have some internal logging, it might be helpful...
If you want to look at logging, Process Monitor may reveal something different in what Opus and Explorer are doing with the same setup.
I don't have a domain setup locally to try on (not sure if Jon does or not) so that might be why I don't see a problem. But I also tend to use pass-through authentication (although not always, and I definitely do get prompted without issue for credentials in Opus if I enter a UNC path which needs one, as I do that a lot on one machine). It could also be other differences, like having the folder tree on or off (usually off here).
The credentials dialog comes from Windows itself, so it's a bit odd really. Unless the multiple dialogs are not actually the first one failing and retrying. It's possible Opus has made multiple requests to the drive in parallel, and Windows isn't intelligent enough to use the correct credentials entered for the first request to satisfy all the others if they've already reached the stage where it thinks credentials are needed. If that theory is right then you should see the same problem if you open two Explorer windows and send them to the same UNC path before typing a password into either of them.