I've been having this issue for quite some time as well, and have been working with a Microsoft Skype for Business (SfB) support engineer on it. To be certain it's the same, or similar-enough issue, here's the setup I've been working with (or against it seems):
* Skype for Business 2016 (any version, including latest in the insider 'Fast Ring' releases)
'Word, Excel, PowerPoint, Outlook and Lync have not been tested on macOS 10.13 High Sierra, and no formal support for this configuration will be provided,' Microsoft wrote in a support document.
* Macbook Pro - Early 2011
* OS X 'El Capitan' - v10.11.6
* Corporate domain is on Azure AD (Active Directory)
* When signing in with the AD account, you are either a) Given a choice between 'Work or school account' or 'Personal account), b) usually automatically redirected to the company branded signon page.
- I have install mac lync to an internal mac that on same network as Lync server. It even join the domain. I can not login I have update to latest version and try add trusted root cert. I able to ping from the mac to the lync server. The lync is in house and not 365 and most solutions I found refer to 365 lync.
- Find answers to Lync 2011 for Mac - sign in failed, service not available from the expert community at Experts Exchange.
If that matches up, the next step is to check the certificate which is on domain's federation services (ADFS) host. This may require getting IT involved to find it, or reviewing the SfB logs.
When you get the host name, for example, 'adfs.mycompany.com', go to that address via HTTPS in Safari. So you'd go to 'https://adfs.mycompany.com'. Once there, click on the lock to the left of the address in the address bar and click the 'Show Certificate' button. Make sure that the selected certificate, in the tree view at the top, is the bottom-most one. The bottom pane should show some brief certificate info (Issued by, Expires, 'This certificate is valid' type message, etc.). Expand the 'Details' section in that lower pane, and look for the 'Signature Algorithm' line, which should be, roughly, the 12th one down. If on that line, you have something similar to 'SHA-512 with RSA Encryption' (forget about the long number afterward), then that is the source of the issue with logging on, and also, activating Office 365 (if you have a company account for it).
Lync Client For Mac
Lync 2011 For Mac Not Signing Instruction
OS X prior to 10.12 (Sierra) does not *natively* support 512 bit certificate signatures. So while browsers and everything else shows that, yes, the certificate is good, valid, unexpired, etc, the low level network stack in OSX, which is used by SfB to initially connect, does not, so it cannot validate that the certificate is valid, thus causing this issue.
Unfortunately, at this time, there doesn't appear to be a way to wedge in support for 512 algorithms in OSX, and that includes forcibly upgrading/linking openssl. The only way I've found to date, to use SfB on anything less than 10.12.x, is to essentially MITM yourself, using a proxy application, such as Charles, which will create its own fake certificate which you must trust, to connect.
Lync 2011 For Mac Not Signing Invalid
I apologize for such a long writeup, but given that despite my hours and hours and hours spent over months of researching the issue, I hope to provide as much useful and helpful information as possible for any future Googlers/Bingers/DuckDuckGoers/etc.