System.Web.Services.Protocols.SoapException: Server was unable to process request. ---> System.Security.SecurityException: Requested registry access is not allowed

Fix:

ASPNET has no rights to write to the event log. Set
\\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Eventlog\Application\RestrictGuestAccess
to 0 (reboot)

ASPNET has also no rights to create event sources. Give ASPNET account
read/write access to
\\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Eventlog (and all
sub keys)

*note - on one install I had to grant the USERS account FULL CONTROL to the event log key, granting it to ASPNET wasn't good enough.

Thank you!  Your two recommended registry permission changes solved a similar error for me.  The error I was receiving was:

System.Security.SecurityException: Requested registry access is not allowed.

Server stack trace:
   at EventLogging.WriteLog(String Info, EventLogEntryType LogType)
   at Microsoft.GreatPlains.eConnect.eConnectMethods.eConnect_EntryPoint(String ConnectionString, ConnectionStringType ConnectionType, String sXML, SchemaValidationType ValidationType, String eConnectSchema)

This error started to occur after I upgraded my dev machine to eConnect 9.0.3 to match the version of an existing client integration.
 

 Once I made the registry permission changes, the registry errors went away and the app started to behave much better.