Since end of September: Mac devices show cert problem on my domain, but not other lets'encrypt encrypted domains

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@Reinhard Welcome to the community!

Your site is serving the "short chain". The shop.bbc.com is serving the "long chain". You must have chosen the alternate chain to do that.

Yours:

Certificate chain
 0 s:/CN=webforum.eu
   i:/C=US/O=Let's Encrypt/CN=R3
 1 s:/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
   i:/O=Digital Signature Trust Co./CN=DST Root CA X3

(Note: The domain you listed shows in the SAN list)

Compare to:

Certificate chain
 0 s:/CN=shop.bbc.com
   i:/C=US/O=Let's Encrypt/CN=R3
 1 s:/C=US/O=Let's Encrypt/CN=R3
   i:/C=US/O=Internet Security Research Group/CN=ISRG Root X1
 2 s:/C=US/O=Internet Security Research Group/CN=ISRG Root X1
   i:/O=Digital Signature Trust Co./CN=DST Root CA X3

Here is a good topic for this: