Corrupted files in the library. If there are files that Seafile can't process, like corrupted PDFs or documents, that might cause the indexer to crash. They could try removing recently added files to see if that helps.
Also, maybe the search index itself is corrupted. Rebuilding the index might fix it. I should mention stopping the service, deleting the index directory, and restarting.
Outdated versions can have bugs. Upgrading Seafile to the latest version could resolve issues that were fixed in updates.
Checking the logs is crucial. The seafile.log or index.log might have specific error messages pointing to the exact problem. Directing the user to look there would help narrow things down.
If the libraries are very large, the indexer might be taking too long or timing out. Increasing timeout settings or resource allocation could help, but that's more of a server-side adjustment.
Database issues could also be a culprit. Maybe running a consistency check with seaf-fsck would identify problems.
Antivirus or other security software might interfere with file indexing. Temporarily disabling them to test could be a troubleshooting step.
Lastly, if all else fails, reinstalling Seafile or reaching out to their support with logs might be necessary