We may have fixed it already. Was your machine upgraded recently? For some reason, crontab doesn’t work right when the default editor is vim.
If you notice this again, see if:
export VISUAL=nvi (or pico, nano, or whatever else you want to use)
works. If you use csh or tsch, use:
setenv VISUAL nvi
If that fixes the problem, contact support and tell them to fix this. We don’t have $VISUAL defined globally, but I believe debian patches stuff to use /usr/bin/editor (which should point to nvi or nano, but sometimes points to vim instead) if $VISUAL or $EDITOR isn’t defined.