OK, that helps a lot already.
The efs.img dump might be all fine and OK, it's actually a problem getting it mounted to "look into it".
Dug around some more and also found this:
2: system error (out of memory, cannot fork, no more loop devices)
"2" was the returncode you got during the check, fits with the verbose error message you get when trying the mount from a shell.
Cool, at least I know where to look, not exately sure why this happens, but we'll find out.
Maybe a busybox thing, maybe I'll bundle one with the app....
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