Hmm...I tried both links and both ones kick me back to the XMB (I'm on 3.40). The problem is that the CORE.SELF file isn't turning into the genesis_plus_gx.SELF. This happened last time, but redownloading solved it. I'll have to do that.
Just for fun, I tried moving the CORE.SELF file out of the directory and hoped it would use the older genesis_plus_gx.SELF from the previous installation. No luck. Then I tried manually renaming the CORE.SELF - nope. Copied the new cores from the new installation and tried to use them with the old version - worked last time, but not now. By new I mean the date stamp is later; "new" doesn't necessarily mean updated.
But as I said, hopefully it's just a bad download. I could blame my phone, but this also happened to me on a PC.
Also, it's nice to see I'm not the only one on a pre-3.55 system.