So I've been having trouble off and on copying files from my Android phone using KDE's file manager. The symptom is that I get a popup message "The process for the mtp protocol died unexpectedly", the phone asks me (again) if I want to allow or deny USB access, and if I attempt the transfer again, the same thing happens. The weird thing was, sometimes I've been able to get it to work without incident, and other times it just fails repeatedly and refuses to work.
Finally figured it out.
If you're using drag-and-drop or copy-paste to copy files from an Android device to a PC using KDE over MTP, the source and destination windows need to be in the SAME INSTANCE (window) of KDE's file manager. You can either: A) select the source files, do a copy, navigate from the source folder to the destination and paste; or B) use separate tabs/panes within a single file manager instance.
If you try to use two separate file manager windows and drag (or copy-paste) between them, you get the above error and the transfer fails.
So I guess the intermittent nature of the problem was based on whether I happened to be using separate file manager windows or not.
All I can say is...