Ok, I'm taking a look at it. At first glance, it looks to me like it isn't specific to v21, but has to do with the lexer choking on a particular Javascript file. What probably happened is the first crash that this dump is for corrupted the tagfile, and after that you couldn't startup. Also means that a re-tag might cause it to fall over again. Or not, it's also possible the bug was triggered by an incomplete statement that was still being typed.
Thanks for the dump, I'm looking into it.