Helping out search engines once again, because this anonymous error is from rclone, and none of the existing resources I found mentioned what turned out to be the cause/solution: I had tried resetting (unsetting) user_allow_other in /etc/fuse.conf (after updating Ubuntu from 24.04 to 26.04, and not remembering why I’d set it before), and that was what the complete non sequitur error message was about.
So: try uncommenting user_allow_other in /etc/fuse.conf to fix this. (The setting takes effect immediately, without needing to reboot or anything.)