Client access issues when mailboxes are mounted on a specific mailbox server

Question

One of our 3 mailbox servers gives us an error when mailbox databases are mounted on it.
All web services are not functioning properly, there is an event log error for each web service web.config file saying file not found, If I open each web.config file and look at the line in question they all point to:
<assemblies>
        <add assembly="Microsoft.Exchange.Security, Version=15.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
      </assemblies>
Does anyone know what this is pointing at or what my server is missing?
It is Exchange 2013 Sp1 RU7 as are my other 2 mbx servers which work fine...
all pre-req's are installed as I double checked and no difference between the 3 mbx servers that I can see...
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Answer 1

IIS error
=== Pre-bind state information ===<o:p></o:p>
LOG: DisplayName = Microsoft.Exchange.Security, Version=15.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35<o:p></o:p>
(Fully-specified)<o:p></o:p>
LOG: Appbase = file:///C:/Program Files/Microsoft/Exchange Server/V15/ClientAccess/Autodiscover/<o:p></o:p>
LOG: Initial PrivatePath = C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\Autodiscover\bin<o:p></o:p>
Calling assembly : (Unknown).<o:p></o:p>
===<o:p></o:p>
LOG: This bind starts in default load context.<o:p></o:p>
LOG: Using application configuration file: C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\Autodiscover\web.config<o:p></o:p>
LOG: Using host configuration file: C:\Windows\Microsoft.NET\Framework64\v4.0.30319\aspnet.config<o:p></o:p>
LOG: Using machine configuration file from C:\Windows\Microsoft.NET\Framework64\v4.0.30319\config\machine.config.<o:p></o:p>
LOG: Post-policy reference: Microsoft.Exchange.Security, Version=15.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35<o:p></o:p>
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework64/v4.0.30319/Temporary ASP.NET Files/autodiscover/b455e468/cbaac37d/Microsoft.Exchange.Security.DLL.<o:p></o:p>
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework64/v4.0.30319/Temporary ASP.NET Files/autodiscover/b455e468/cbaac37d/Microsoft.Exchange.Security/Microsoft.Exchange.Security.DLL.<o:p></o:p>
LOG: Attempting download of new URL file:///C:/Program Files/Microsoft/Exchange Server/V15/ClientAccess/Autodiscover/bin/Microsoft.Exchange.Security.DLL.<o:p></o:p>
LOG: Attempting download of new URL file:///C:/Program Files/Microsoft/Exchange Server/V15/ClientAccess/Autodiscover/bin/Microsoft.Exchange.Security/Microsoft.Exchange.Security.DLL.<o:p></o:p>
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework64/v4.0.30319/Temporary ASP.NET Files/autodiscover/b455e468/cbaac37d/Microsoft.Exchange.Security.EXE.<o:p></o:p>
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework64/v4.0.30319/Temporary ASP.NET Files/autodiscover/b455e468/cbaac37d/Microsoft.Exchange.Security/Microsoft.Exchange.Security.EXE.<o:p></o:p>
LOG: Attempting download of new URL file:///C:/Program Files/Microsoft/Exchange Server/V15/ClientAccess/Autodiscover/bin/Microsoft.Exchange.Security.EXE.<o:p></o:p>
LOG: Attempting download of new URL file:///C:/Program Files/Microsoft/Exchange Server/V15/ClientAccess/Autodiscover/bin/Microsoft.Exchange.Security/Microsoft.Exchange.Security.EXE.<o:p></o:p>
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