via: https://github.com/magento/magento2/issues/5836#issuecomment-329783585
hi there!
we can confirm we were able to reproduce the issue on 2.1.8 too (after upgrading from 2.1.5).
truncating the table cron_schedule, setting the indexer on “update on save” and re-setting them on “update on schedule” and finally clearing page_cache fixed the issue once again for us (at the first try).
For completeness’ sake: we still run with the following settings:
1 Stores -> Configuration -> Advanced -> System -> Cron (Scheduled Tasks) -> Cron configuration options for group:index -> Use Separate Process -> SET TO “YES”
2 Stores -> Configuration -> Advanced -> System -> Cron (Scheduled Tasks) -> Cron configuration options for group:default -> Use Separate Process -> SET TO “NO”)
cheers!