Here is a video of a bug with Dopus trying to read SD card folder on attached mobile device. The first 2 attempts in the video are Dopus and the 3rd attempt is with Total Commander (just to show that there is no problem with the connection).
You also have a script error blinking in your status bar, which should be looked into before anything else. If a script is going wrong it could break all kinds of things.
Script error is from Ebook Columns. comes up when trying to work with ebooks on SD card
Disable it and make sure there are no more script errors appearing in the log.
If you still have problems with MTP, see the FAQ I linked.
I'd also recommend fully exiting Opus (File > Exit Directory Opus) and also closing anything else that's accessing the device before trying again. MTP is very sensitive to multiple things trying to access it at once, and once it goes wrong in one process or another it can stay wrong until it's restarted.
(The FAQ has lots of other suggestions.)
toggling Preferences / Miscellaneous / Advanced: mtp_enable does not work will try the ftp recommendation.
ftp is slow and had a timeout error from Dopus when trying to read a folder with 45 files in it. Best route for me is to use Total Commander.
Taking the SD card out and using an SD card reader also works but takes longer.
FTP works flawlessly with my phone. Maybe a firewall issue or something.
Try a different FTP server on the phone. They differ substantially in performance, especially when handling SD cards.