All my post URLs suddenly ended with /s

Check your permalink structure.

Install and check if these /s remain when all plugins and your theme are deactivated. (Health Check lets you try that without anything changing for other visitors to your site.) If it goes away without all the plugins, try and activate one plugin at a time while revisiting a page on the front and checking what URL:s get fed out. Hopefully, you’ll this way be able to find the culprit.

I check the permalink and find it is set to custom structure with ending /s, don’t know why? But I reset it to post name.

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