Here is the error message but the user was migrated successfully and shows up in the 2013 server database.
Request
Url: https://mail1.onshoreit.local:444/owa/
User host address: fe80::adc9:f898:18f1:58e9%12
User: Daniel Flick
EX Address: /o=OnshoreIT/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=Daniel Flickabe
SMTP Address: dflick@onshoreit.net
OWA version: 15.0.516.30
Exception
Exception type: Microsoft.Exchange.Data.Storage.NotSupportedWithServerVersionException
Exception message: The mailbox of user dflick@onshoreit.net that is located on a server that is running version 14 can't be opened on a server that is running version 15.
Call stack
Microsoft.Exchange.Data.Storage.MailboxSession.InternalCreateMailboxSession(LogonType logonType, ExchangePrincipal owner, DelegateLogonUser delegatedUser, CultureInfo cultureInfo, String clientInfoString, IBudget budget, Action`1 initializeMailboxSession,
InitializeMailboxSessionFailure initializeMailboxSessionFailure)
Microsoft.Exchange.Data.Storage.MailboxSession.CreateMailboxSession(LogonType logonType, ExchangePrincipal owner, DelegateLogonUser delegateUser, Object identity, OpenMailboxSessionFlags flags, CultureInfo cultureInfo, String clientInfoString,
PropertyDefinition[] mailboxProperties, IList`1 foldersToInit, GenericIdentity auxiliaryIdentity, IBudget budget)
Microsoft.Exchange.Data.Storage.MailboxSession.ConfigurableOpen(ExchangePrincipal mailbox, MailboxAccessInfo accessInfo, CultureInfo cultureInfo, String clientInfoString, LogonType logonType, PropertyDefinition[] mailboxProperties, InitializationFlags
initFlags, IList`1 foldersToInit, IBudget budget)
Microsoft.Exchange.Data.Storage.MailboxSession.Open(ExchangePrincipal mailboxOwner, ClientSecurityContext clientSecurityContext, CultureInfo cultureInfo, String clientInfoString)
Microsoft.Exchange.Clients.Owa.Core.OwaClientSecurityContextIdentity.CreateMailboxSession(ExchangePrincipal exchangePrincipal, CultureInfo cultureInfo, HttpRequest clientRequest)
Microsoft.Exchange.Clients.Owa.Core.UserContext.CreateMailboxSession(OwaContext owaContext)
Microsoft.Exchange.Clients.Owa.Core.UserContext.Load(OwaContext owaContext)
Microsoft.Exchange.Clients.Owa.Core.RequestDispatcher.CreateUserContext(OwaContext owaContext, UserContextKey userContextKey, UserContext& userContext)
Microsoft.Exchange.Clients.Owa.Core.RequestDispatcher.PrepareRequestWithoutSession(OwaContext owaContext, UserContextCookie userContextCookie)
Microsoft.Exchange.Clients.Owa.Core.RequestDispatcher.AcquireAndPreprocessUserContext(OwaContext owaContext, HttpRequest request)
Microsoft.Exchange.Clients.Owa.Core.RequestDispatcher.InternalDispatchRequest(OwaContext owaContext)
Microsoft.Exchange.Clients.Owa.Core.RequestDispatcher.DispatchRequest(OwaContext owaContext)
Microsoft.Exchange.Clients.Owa.Core.OwaRequestEventInspector.OnPostAuthorizeRequest(Object sender, EventArgs e)
Microsoft.Exchange.Clients.Owa.Core.OwaModule.OnPostAuthorizeRequest(Object sender, EventArgs e)
System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)