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https://www.vspbreda.nl/nl/exchange/exchange-2013/solved-office-365exchange-unable-to-activate-hybrid-deployment-content-was-blocked-because-it-was-not-signed-by-a-valid-security-certificate-for-more-information-see-about-certificat/
[Solved] Office 365/Exchange : Unable to activate hybrid deployment (Content was blocked because it was not signed by a valid security certificate. For more information, see “About Certificate Errors” in Internet Explorer Help )