It could be something in your config pointing to an old UNC path, maybe. For example, in the Favorites list, or a toolbar button that goes to that path or that uses an icon or program from it.
Old File Collections (e.g. Find Results) pointing to the old paths could be involved (although I think we avoid accessing those these days, it's worth clearing out old, unneeded collections in case they are involved).
Check if you have any other Opus windows open, as they can persist across restarts (depending on config) and some people have accumulated them without realising.
If there are shortcuts in the folders you're viewing, and one of those points to an old UNC path, those might be involved as well, although I'm not sure.
Process Monitor can do that. If you filter the path to ones starting with \\ then it should show all the UNC paths that are being accessed and which process is accessing them.
