My import worked before but now fails after upgrading WordPress / WooCommerce / a plugin: what changed?

Read the actual error and the failing stage before changing settings. Checks include:

1. MySQL max_allowed_packet. If the error says a packet is too large, review the server limit and the size of the affected data (see MySQL stopped mid-import).
2. innodb_log_file_size changes between MySQL versions. Check the settings after a database upgrade. See Database tuning.
3. Plugin conflict; a new plugin hooks save_post and crashes. The save-posts tool in SSI (wp ssi <id> save-posts) re-fires this hook for every post; check the CLI output for fatal errors mentioning a specific plugin file.
4. Super Speedy Search updated to 5.80. Search renamed its rebuild command, settings page and rebuild service in 5.80. Super Speedy Imports 3.29 and later use the new names, and the old ones for earlier Search versions; on an older Super Speedy Imports the After Import Search rebuild stops working. Update Super Speedy Imports.
5. WooCommerce lists Super Speedy Imports as incompatible with HPOS. Super Speedy Imports declares High-Performance Order Storage compatibility from 3.25. Update it.

If the message matches one in An error that updating fixes, updating Super Speedy Imports is the fix.

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Written for Super Speedy Imports 3.40.0.