Bruenor emailed me and helped me figure out that my device was marking some directories as inaccessible, but not others (even though they in fact were). I was allowed to point my configuration to a directory that is now forbidden, which caused a silent crash.
It works perfectly now!
Google is becoming more and more like Apple all the time. Just last night I was trying to use GDrive to transfer whole directories to my device, but uploading/downloading whole directories is forbidden on mobile, for my own good I guess?
I keep holding my breath for the Linux smartphone ecosystem to mature. One of these years . . .