The problem you are reporting is very much likely because the processor is not supported by the BIOS, as you have discovered. The processor my work, to a degree, but be prepared for sporadic errors/issues, including the L2 cache being misreported.
Your best approach is to get one of the supported processors.
Also, this board is probably 12 years old. It is long past its life. I would not be surprised if the board started exhibiting signs of failure.
It is also possible that the BIOS simply has a bug and is incorrectly identifying the L2 cache size. I have seen this more than once - and not simply because this isn't a processor on the support list. If you are running on the latest BIOS release (though that's a relative term for a 12-year old board ), then there are no BIOS releases available with this fixed - nor will there for a board that was end-of-lifed so long ago. Does this affect operation? Unlikely. Does this affect performance? No.