Thanks for the hint.
Unfortunately, the BIOS of my box (it's a Neoware box) does just offer 
Normal, LBA, Large and Auto for the hard drive. None of the options work.
It's a IDE2Flash adaptor, and the error message says:

"FATAL ERROR
The device that contains the configuration file (config.xml) could not be
found. m0n0wall cannot continue booting."


Chris Buechler schrieb:
> That message is generally solved by switching the drive to CHS mode in
> the BIOS, but why that would have changed from one release to another
> I don't know.
>
> -Chris