When Leo says, "Try double-clicking in the file display instead of using the folder tree," he means he wants you to double-click screenshots in the pane where the files would be listed. Or to put it another way, do not use the tree to do your navigation.
@Leo, it appears in his original screenshot as if the file pane itself is not even populating with folder names. Going by his first screenshot (upper-left portion), what exactly do you want him to double-click?
As for FTP - with all due respect and definitely no offense meant: reason for not getting into this FTP matter is that I am not familiar with it. As far as I remember I never used FTP. That aside, whilst checking Opus Help file, I believe(..) it is mostly related to websites.
At first glance I could not find anything relating to connecting to an Android device. Maybe it is easy, I don't know.
Thing is that at some point one has to consider 'Should I spend time on reading all this stuff and/or maybe need install I don't know what - try this and ask that in order to get it working, just for the occasional use'. In case of regular or daily use, I agree, it is different.
Again, no offense meant.
The issue should be fixed in the current version, per Jon's post above yours.
If you still have problems, try toggling mtp_enable to False, which will delegate more of the work to the Windows shell. Although I would still recommend FTP as MTP doesn't always work well for long even in Explorer, in my experience.
Jon's post was in June. I am using v12.16.5, not the very latest one, but I believe fixes were in 12.15.1 and 12.15.3 (?)
I toggled MTP_Enabled from False to True and back to False, File->Exiting in between.
It made no difference.
Below for information.