I'd encountered exactly the same problem(-ERR aack, child crashed) with vchkpw using LDAP as backend. I'd tried many different configurations and finally it seem like I can get around this problem.
The solution is configure vpopmail with --enable-clear-passwd=n parameter. My configure statement looks like:
./configure --enable-ldap=y --enable-clear-passwd=n
I have no idea how this fix the problem, but it's something with the authentication.
Hope this help,
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