This is a very common issue and comes with a multiple errors, I spoke in my previous article about one of this errors. Today we have the same error but with a little different symptom.
Lync Mobile users get the following Error message “can’t connect to the server. It may be busy or temporarily unavailable” while trying to connect, in addition the S4B mobile clients have the same symptom as we have a hybrid deployment with Skype for Business Online,and the new in this issue is that this problem occurs at irregular intervals
Quite simply the first thing we should think about it when we find that this problem is repeated irregularly, is to review the (Autodiscover and MCX) directories settings over IIS web component on the Frontend servers in case you have a multiple frontend servers.
In my case I have two frontend servers one of them has lost the autodiscover and MCX directories.
you need to rebuild the autodiscover and MCX directories on defected server by the following steps;
Log on to the computer as a user who is a member of the CsAdministrator group.
Download the latest installation package from the Microsoft Download Center and extract it to the hard disk.
Copy McxStandalone.msi to C:\ProgramData\Microsoft\Lync Server\Deployment\cache\4.0.7577.0\setup.
Open the command prompt: Click Start, click in the search box, type cmd, and then press ENTER.
At the command prompt, run C:\Program Files\Microsoft Lync Server 2010\Deployment\Bootstrapper.exe.
If you run Bootstrapper.exe from Lync Server Management Shell, you must prepend the path with a period (.) and enclose the path in quotation marks ("). For example: ."C:\Program Files\Microsoft Lync Server 2010\Deployment\Bootstrapper.exe".