Could you share a screenshot of the error message?
Sure, the SIP address could start with capital letters.
I tested this in my lab. I changed user’s email address and UPN starting with capital letters. Then, re-enable this user in Skype for Business Server Control Panel. It will take up several minutes to take effect. Then, I log in client successfully as below:
Please note before taking effect, it will show the following error message when you try to log in Skype for Business.
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