Hi @Alper Günay
Thanks for your feedback above which shared more information and glad to know that your issue is resolved now! Since our forum has the policy that The question author cannot accept their own answer. They can only accept answers by others, and according to the scenario introduced here: Answering your own questions on Microsoft Q&A
I would make a brief summary of this post so that other forum members could easily find useful information here:
[S4B Client signing-in "There was a problem verifying the certificate from the server" - Summary]
Issue Symptom: Domain name and sip domain has changed from olddomain.com to newdomain.com because of company name changing. Exchange side is ok. no problem for accessing mailboxes. But We have an issue. client is getting error when client is signing into S4B client, "There was a problem verifying the certificate from the server".
Solution: Oauth certificate was expired on Front end server.We renewed oauth certificate and published topology. After that, client was able to sign-in Skype for Business
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