Synology
I ran into an issue with Universal Search on my Synology DS415+, where all search attempts resulted in this error message: "The operation Failed. Please log in to DSM again and retry." Synology support informed me that there was a corrupted search index. To resolve this, they deleted all search indexes and started a rebuild of my indexes. Here's how to do that from an SSH shell:
### SSH as an admin user, or sudo to root
for db in /var/packages/SynoFinder/etc/appindex.db/*; do echo -n "$db : "; /var/packages/SynoFinder/target/tool/synoelastic -a check -d $db; done
rm -f /var/packages/SynoFinder/etc/appindex.queue*
/var/packages/SynoFinder/target/tool/appindex -a reindex