CERTIFICATE CHAINS
An incomplete certificate chain works perfectly in your browser and fails for other people, which makes it the hardest certificate problem to notice and one of the easiest to fix.
This is the short version: what a chain is, why yours looks fine, and how to install one properly.
What this covers
Chapter 1 of 5
Leaf, intermediate, root, and who holds which.
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Chapter 2 of 5
Cached intermediates, and the person least able to notice.
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Chapter 3 of 5
Fresh devices, API clients, payment integrations.
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Chapter 4 of 5
Full chain files, and where the configuration differs.
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Chapter 5 of 5
From a machine that has never visited your site.
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Check yours
Everything above, checked on your domain in about ten seconds. Free, no account.