How to backup files when FP1(U) doesn't start any more?

Just to give an update on what happened:
I reinstalled/updated the OS to the 1.8.7 version (for the original storage layout), but I had the same problem. I then tried with the 1.8.5, 1.6, 1.3, 1.1 and 1.0 versions, all with the same result. @Stefan suggested that parts of my data partition might be corrupted. I was originally going to test with the OS versions for the new storage layout for the sake of completeness, but applied the 1.8.7 layout upgrade first. (I forgot that this would wipe all the data anyway, so we’ll never know if the other updates would have fixed it without data loss or not.) This sadly meant a full data wipe, as expected, but at least the phone now boots properly and in general seems to work just fine.

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