Hello @Marek G ,
Here's a KB for the same error, although in a different product:
KB-2860844
Use the steps at the bottom of the page to check the rangeUpper property of "CN=ms-Exch-Smtp-TLS-Certificate" using ADSI Edit.
According to documentation, Exchange 2019 should have this value set at 1024 by default:
Active Directory schema changes in Exchange Server - Attributes modified by Exchange 2019 RTM
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