There is no older firmware then the 1.7.5rc, the official links expired and those files are unavailable. From the remaining options in the recovery menu I did a factory reset and when that didn't work I deleted the cache, this also didn't result in a system that gets through the "Starting Up" phase.
So it seems the official firmware update from sd card left my device unusable. If only I could restore an older (and compatible) image in some way.
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