What's the detailed version of your Exchange servers? You can check with the following command:
Get-ExchangeServer | Format-List Name,Edition,AdminDisplayVersion
Do you get the same error alert when send a new message or reply to other messages without digitally signed?
Is there a ribbon icon when you reply the message? Like this:
Please check the S/MIME settings in your OWA. Options > Mail > S/MIME, make sure "Add a digital signature to all messages I send" is not selected:
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