I'm having frequent problems with DOPUS running into permission or "file not found" errors when copying files/folders. This seems to mostly affect my network drives, and when I use the standard W10 File Explorer I do not have any issues. In addition, I've noticed in some transfers that DOPUS won't use its built in file transfer tool but it resorts to the native explorer one, and locks up the DOPUS lister until the transfer is complete.
Could you post a screenshot of the error messages? The look of the dialog can tell us where it is coming from, in addition to the full detail.
Process Monitor logs of what happens and details of which paths the errors occurred with may also help point to what is going wrong.
What kind of network drives are they?
That sounds like the drive you're copying to (or from) isn't a normal directory, and is instead accessed by a custom user interface provided by third party software (e.g. software that came with a non-standard type of NAS perhaps). Those are very rare these days but do still exist. In those cases, Opus has little to do with what's happening and just acts as a container for the sub-window of some other software.
If you post a screenshot we should be able to tell if that is what's happening, since custom UIs tend to look slightly different.
It could also be something else, like a custom drop handler has been installed, which would intercept file copies and run them via 3rd party code while leaving the view of the directories alone, but that is also very rare these days.