If you are familiar with the android usb debugging option, especially adb, I’d be interested in the output of
$ adb shell df -h /storage/emulated/
$ adb shell
FP3:/ $ df -h /storage/emulated/Filesystem Size Used Avail Use% Mounted on
/dev/fuse 48G 39G 8.6G 82% /storage/emulated
On my end the UI refers to 9.3GB of free storage while the console tool reports 8.6GB. It would be interesting if there is some app side cache for a messenger or so which is not accounted in the UI storage statistics which would be filling up your 10GB of free space into the region of ~5GB reported in your own reference which is a “confirmed” quota for this error message. (Iam too lazy to calculate if this discrepancy on my FP3 is due to distinguishing effect GB ↔ GiB or that data placed on FS is aranged in blocks, thus taking more space)