[389-users] 1.3.6 dirsrv crash: ERR - valueset_value_syntax_cmp - slapi_attr_values2keys_sv failed for type lastUpdated

OS: CentOS Linux release 7.4.1708 (Core) 
dirsrv: 1.3.6.1 B2017.249.1616

I've had two of these running in multi-master replication for a week now with no
issues, but last night they both crashed at the same time and there were a lot of these
just before they died:

[06/Oct/2017:22:30:41.990009449 -0700] - ERR - valueset_value_syntax_cmp -
slapi_attr_values2keys_sv failed for type lastUpdated
[06/Oct/2017:22:30:41.991965822 -0700] - ERR - valueset_value_syntax_cmp -
slapi_attr_values2keys_sv failed for type lastUpdated
[06/Oct/2017:22:30:41.993908534 -0700] - ERR - valueset_value_syntax_cmp -
slapi_attr_values2keys_sv failed for type lastUpdated

When I try to start either server now, I get the usual recovery messages and then a bunch
of these errors and a crash. I've checked as many things as I can think of, including
dse.ldif, which is fine.

Unrelated probably, but annoying, my error logs are also filling up with lots of these:
[06/Oct/2017:21:51:16.987020789 -0700] - ERR - attrlist_replace - attr_replace
(nsslapd-referral, ldap://ldap2.arizona.edu:389/dc%3Deds%2Cdc%3Darizona%2Cdc%3Dedu)
failed.