If you have trouble accessing https://server/oma, check that your exchange-oma virtual directory is intact.

You may get the error "A System error has occurred while processing your request. Please try again. If the problem persists, contact your administrator"

You may also notice a system event 1503 logged from source MSExchangeOMA with the following description:

Event Type:    Error
Event Source:    MSExchangeOMA
Event Category:    (1000)
Event ID:    1503
Date:        27/04/2009
Time:        4:45:19 PM
User:        N/A
Computer:    SERVER
Description:
An unknown error occurred while processing the current request:
Message: Index was out of range.  Must be non-negative and less than the size of the collection.
Parameter name: startIndex
Source: mscorlib
Stack trace:
   at System.Globalization.CompareInfo.IndexOfString(Void* pSortingTable, Int32 win32LCID, String source, String value, Int32 startIndex, Int32 count, Int32 options)
   at System.Globalization.CompareInfo.IndexOf(String source, String value, Int32 startIndex)
   at Microsoft.Exchange.OMA.Preferencing.AdUserObject.get_globalWirelessEnable()
   at Microsoft.Exchange.OMA.Preferencing.AdUserObject.get_UserIsWirelesslyEnabled()
   at Microsoft.Exchange.OMA.Preferencing.OmaUserInfo.get_UserIsWirelesslyEnabled()
   at Microsoft.Exchange.OMA.UserInterface.Global.Session_Start(Object sender, EventArgs e)

Message: Exception of type Microsoft.Exchange.OMA.DataProviderInterface.ProviderException was thrown.
EventMessage:
UserMessage: A System error has occurred while processing your request. Please try again. If the problem persists, contact your administrator.
Source: Microsoft.Exchange.OMA.UserInterface
Stack trace:
   at Microsoft.Exchange.OMA.UserInterface.Global.Session_Start(Object sender, EventArgs e)
   at System.Web.SessionState.SessionStateModule.RaiseOnStart(EventArgs e)
   at System.Web.SessionState.SessionStateModule.CompleteAcquireState()
   at System.Web.SessionState.SessionStateModule.BeginAcquireState(Object source, EventArgs e, AsyncCallback cb, Object extraData)
   at System.Web.AsyncEventExecutionStep.System.Web.HttpApplication+IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.

Solution:

Check that you have an exchange-oma virtual directory in the IIS Manager console. If you don't, follow the instructions in Microsoft KB article 817379 to recreate it.