There are many reasons why this error may occur. First, this error is an issue associated with the DKIM-Signature header rather than the DKIM key. The header probably does not comply with the RFC 6376 rules hence raising this error.
Make sure your DKIM signature is valid. Then, try with other email providers to check whether you are getting the same error. If not, contact the email provider in question and get their support in resolving the issue, as the error is probably related to the email provider. The root cause can vary based on the tools and third-party technologies you are using.
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