Error creating Room Resource Mailbox

Hi
When creating a Room Resource Mailbox in Exchange 2013 I experience the following

warning
This mailbox was created successfully, but some properties weren't saved. Close this message, then in the Mailboxes list, double-click this mailbox, set the properties, and click Save.

error
Cannot open mailbox /o=Company/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Configuration/cn=Servers/cn=MailboxServer/cn=Microsoft System Attendant.

critical error*
The web page isn't loading correctly. Please reload the page by refreshing your browser.Technical Information: Invalid target element for this operation.The entire error report is listed below: <at the end of this post>

When I edit the mailbox, I notice that under booking delegates there are 3 options

- Use customized setting to accept or decline booking requests
- Accept or decline booking requests automatically
- Select delegates who can accept or decline booking requests

- Use customized setting to accept or decline booking requests is a deprecated feature, so when I select:
- Accept or decline booking requests automatically, the issue is resolved

Question is why is this happening?
Thanks

*Client Information
User Agent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; .NET CLR 1.1.4322; .NET4.0C; .NET4.0E; InfoPath.3; MS-RTC LM 8)
CPU Class: x86
Platform: Win32
System Language: en-gb
User Language: en-GB
CookieEnabled: true
-----------------
Exception Details
-----------------
Date: Sat Mar 29 14:14:47 UTC 2014
Message: Invalid target element for this operation.
Url: https://mail.hds.int/ecp/15.0.775.37/scripts/wizardproperties.js
Line: 1

Call Stack
----------


Dump Event
----------
    errorMessage = Invalid target element for this operation.
    errorUrl = https://mail.hds.int/ecp/15.0.775.37/scripts/wizardproperties.js
    errorLine = 1
    errorCharacter = 81219
    errorCode = 0
    actionURL =
    altKey = false
    altLeft = false
    behaviorCookie = 0
    behaviorPart = 0
    bookmarks = null
    boundElements = [object HTMLCollection]
    button = 0
    buttonID = 0
    cancelBubble = false
    clientX = 441
    clientY = 160
    contentOverflow = false
    ctrlKey = false
    ctrlLeft = false
    data =
    dataFld =
    dataTransfer = null
    fromElement = null
    keyCode = 0
    nextPage =
    offsetX = 0
    offsetY = 0
    origin =
    propertyName =
    qualifier =
    reason = 0
    recordset = null
    repeat = false
    returnValue = true
    screenX = 934
    screenY = 433
    shiftKey = false
    shiftLeft = false
    source = null
    srcElement = null
    srcFilter = null
    srcUrn =
    toElement = null
    type = error
    url =
    wheelDelta = 0
    x = 0
    y = 0
    getAttribute =
function getAttribute() {
    [native code]
}

    removeAttribute =
function removeAttribute() {
    [native code]
}

    setAttribute =
function setAttribute() {
    [native code]
}


Detailed Call Stack
-------------------

shapi