Okay, the voltages seem to be pretty much the problem.
One hint: RBE v1.15 (and any future versions, of course) has combo boxes for voltage selection in the clock settings tab instead of the old textboxes. Those combo boxes contain a list of the voltages the BIOS itself reports to being able to set on the card. Be sure to use those voltages only. This seems to solve the problem.
In fact, this is no real restriction: Imagine a BIOS with a maximum voltage of, say, 1.237 volts in the voltage table. Maybe you felt like entering, say, 1.4 volts in order to gain a maximum voltage out of the card. I did that myself as I didn't know about the voltage table yet...
For Catalysts previous than v8.8, this was no problem. It assumed an error in the powerplay table but it was clear what to do: Order the voltage controller on the card to set the maximum voltage. That was 1.237 volts.
In other words: Entering higher voltages (or lower ones, also) than the boundaries of the voltage table allow was pointless from the very beginning because there is a hardware limitation: The controller could simply select the voltages that are hard-wired on the card anyway. So be sure to set voltages allowed by the combo list only. Still, you can enter your own values. But to quote myself: This is pointless. It had no effect before and produces an error now with newer drivers...
Think about it: Of course, this is the cause for old modded BIOSes (that worked with Catalyst v8.7 and previous) not to work anymore now. All you have to do is to re-mod your BIOS observing the voltage table (or easier, selecting voltages from those combo boxes).
Remember one thing: In case you choose to overclock your card using Overdrive, you don't have to care about this. Even if you modify your BIOS using RBE's Overdrive increasing hack, you don't. Overdrive selects the voltages on its own, then.