SBS 2011: Autodiscover fails with HTTP error 500

I have configured autodiscover for a SBS 2011. It has been working nice, but now it suddenly doesn't work.

External domain is configured with a SRV record: _autodiscover._tcp.domain.tld pointing to port 443 remote.domain.tld.

When autodiscover reaches the point it has to contact https://remote.domain.tld//Autodiscover/Autodiscover.xml it fails with a HTTP 500 error "You do not have permission to view this directory or page."
Same happens if I try to login to the webpage from IE (I'm prompted 3 times for username/password).

Also same error if I run PowerShell command "test-outlookwebservices" (See log below) or Autodiscover test fromhttps://www.testexchangeconnectivity.com

Anyone got a clue how to fix this error ? This was working perfectly but suddenly stopped working after server reboot...

[PS] C:\>test-outlookwebservices -Identity:lbk@domain.tld

RunspaceId : 8851b6d5-4f07-404d-874e-b4463611d5d5
Id         : 1019
Type       : Information
Message    : A valid Autodiscover service connection point was found. The Autodiscover URL on this object ishttps://remote.domain.tld/Autodiscover/Autodiscover.xml.

RunspaceId : 8851b6d5-4f07-404d-874e-b4463611d5d5
Id         : 1013
Type       : Error
Message    : When contacting https://remote.domain.tld/Autodiscover/Autodiscover.xml received the error The remote server returned an error: (500) Internal Server Error.

RunspaceId : 8851b6d5-4f07-404d-874e-b4463611d5d5
Id         : 1023
Type       : Error
Message    : The Autodiscover service couldn't be contacted.

RunspaceId : 8851b6d5-4f07-404d-874e-b4463611d5d5
Id         : 1113
Type       : Error
Message    : When contacting https://server.internal.local:443/Autodiscover/Autodiscover.xml received the error. The remote server returned an error: (500) Internal Server Error.

RunspaceId : 8851b6d5-4f07-404d-874e-b4463611d5d5
Id         : 1123
Type       : Error
Message    : The Autodiscover service couldn't be contacted.

RunspaceId : 8851b6d5-4f07-404d-874e-b4463611d5d5
Id         : 1013
Type       : Error
Message    : When contacting https://remote.domain.tld/EWS/Exchange.asmx received the error Client found response content type of '', but expected 'text/xml'.
             The request failed with an empty response.

RunspaceId : 8851b6d5-4f07-404d-874e-b4463611d5d5
Id         : 1025
Type       : Error
Message    : [EXCH] Error contacting the AS service at https://remote.domain.tld/EWS/Exchange.asmx. Elapsed time was 839 milliseconds.

RunspaceId : 8851b6d5-4f07-404d-874e-b4463611d5d5
Id         : 1026
Type       : Success
Message    : [EXCH] Successfully contacted the UM service at https://remote.domain.tld/EWS/Exchange.asmx. The elapsed time was 106 milliseconds.

RunspaceId : 8851b6d5-4f07-404d-874e-b4463611d5d5
Id         : 1113
Type       : Error
Message    : When contacting https://server.internal.local/ews/exchange.asmx received the error Client found response content type of '', but expected 'text/xml'.
             The request failed with an empty response.

RunspaceId : 8851b6d5-4f07-404d-874e-b4463611d5d5
Id         : 1125
Type       : Error
Message    : [Server] Error contacting the AS service at https://server.internal.local/ews/exchange.asmx. Elapsed time was 21 milliseconds.

RunspaceId : 8851b6d5-4f07-404d-874e-b4463611d5d5
Id         : 1126
Type       : Success
Message    : [Server] Successfully contacted the UM service at https://server.internal.local/ews/exchange.asmx. The elapsed time was 22 milliseconds.