CRM Outlook Client Installation fails

Hi there

We are using CRM Online.

I've installed successfully many times the Client on colleagues PCs.

Since few weeks I run into the following Problem:

• Client installs properly
• When I'm asked to add the organization I Chose "CRM Online"
• the System starts configuring but stops at 50% with the follwing error "Ungueltiges Argument"
  I guess in English the message will be "Invalid Argument"

The last lines of the error log look like this:

13:35:04|Verbose| Method exit: Microsoft.Crm.Application.Outlook.Config.OutlookCRMDatastoreInstaller.GetCrmStoreServiceId
13:35:04|Verbose| Method exit: Microsoft.Crm.Application.Outlook.Config.OutlookCRMDatastoreInstaller.DeleteV4CrmDatastore
13:35:04|Verbose| Method entry: Microsoft.Crm.Application.Outlook.Config.OutlookConfigurator.InitializeMapiStoreForFirstTime
13:35:09|  Error| Exception : context    bei Microsoft.Crm.Metadata.MetadataCache.GetInstance(IOrganizationContext context)
   bei Microsoft.Crm.Application.ControlDescriptor.get_Metadata()
   bei Microsoft.Crm.Application.FormDescriptor.GetChildColumns()
   bei Microsoft.Crm.Application.FormDescriptor.AddHiddenControls(EntityMetadata entityMetadata)
   bei Microsoft.Crm.Application.FormDescriptor.Initialize(EntityMetadata entityMetadata, XmlNode formXml, Guid formId, FormType formType)
   bei Microsoft.Crm.Application.FormDescriptor..ctor(EntityMetadata entityMetadata, XmlNode formXml, Guid formId, FormType formType, Boolean isUserForm, Boolean isActive, IOrganizationContext context)
   bei Microsoft.Crm.Application.Forms.FormDescriptorLoader.LoadCacheData(FormDescriptorCacheKey key, IOrganizationContext context)
   bei Microsoft.Crm.Caching.CrmSharedMultiOrgCache`2.LookupEntry(TKey key, IOrganizationContext context)
   bei Microsoft.Crm.Application.Outlook.Config.OutlookConfigurator.InitializeMapiStoreForFirstTime()
   bei Microsoft.Crm.Application.Outlook.Config.OutlookConfigurator.Configure(IProgressEventHandler progressEventHandler)

Any ideas what goes wrong here?

Thanks in advance for any hint or help!

Regards, Helmut
   bei Microsoft.Crm.Application.Outlook.Config.ConfigEngine.Configure(Object stateInfo)
13:35:09|  Error| Exception : Der Wert darf nicht NULL sein.
Parametername: context