I know nothing of LIDS and next to nothing about getting a recompiled kernel to work if it somehow doesn't (the god of kernel recompilation generally smiles warmly on me), but...
.... are you sure you're using the right version? You might accidentally be using an unstable or test version. According to lids.org, the following is the stable release for the 2.4.21 kernel:
http://www.lids.org/download/v2.4/2....-2.4.21.tar.gz
Good luck!
-zsejk