I had the same "garbage" screen after upgrading my mk1 from Beta 12C to RC1 (both were developer versions). I don't have a custom logo, and I did unplug my empeg while upgrading.

(list 6284, S/N 00299, 4 gig blue)
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(list 6284, Mk1 S/N 00299 4GB blue [sold]. Mk2 S/N 080000094 20GB blue)