Error Message

Method #1.

If the Mail app is deployed with the following command:

$Mbx = get-mailbox "mailbox"
New-App -mailbox $Mbx.LegacyExchangeDN -Url `
    ("https://EV_server.domain.com/EnterpriseVault/OfficeMailAppManifest.aspx?LegacyMbxDn=" +
    [System.Web.HttpUtility]::UrlEncode($Mbx.LegacyExchangeDN))

The following error occurs:

The app couldn't be downloaded.
    + CategoryInfo          : InvalidData: (:) [New-App], LocalizedException
    + FullyQualifiedErrorId : [Server=EX1,RequestId=e61f89f6-f16b-4f07-b9e1-9ba9b6beedb6,TimeStamp=7/14/2022 10:05:33 
PM] [FailureCategory=Cmdlet-LocalizedException] 3E374F34,Microsoft.Exchange.Management.Extension.NewApp

Method #2

If the mail app is deployed with the following commands:

Add-Type -AssemblyName System.Web
$Mbx = get-mailbox "mailbox"
$uri = new-object system.uri(
    "https://EV_server.domain.com/EnterpriseVault/OfficeMailAppManifest.aspx?LegacyMbxDn=" + 
    [System.Web.HttpUtility]::UrlEncode($Mbx.LegacyExchangeDN))
$webclient = New-Object Net.Webclient
$webClient.UseDefaultCredentials = $true
try
{
    $bytes = $webclient.DownloadData($uri)
    New-App -mailbox $Mbx.LegacyExchangeDN -FileData $bytes
}
catch [Net.WebException]
{
    [Net.HttpWebResponse] $webResponse = [Net.HttpWebResponse]$_.Exception.Response;
    Write-Warning $webResponse.StatusDescription
}

This error happens:

Write-Warning : Cannot bind argument to parameter 'Message' because it is null.
At line:1 char:226
+ ... $_.Exception.Response; Write-Warning $webResponse.StatusDescription;}
+                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [Write-Warning], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationErrorNullNotAllowed,Microsoft.PowerShell.Commands.WriteWarnin
   gCommand

If the try/catch block is removed from the above command, the underlying error is revealed:

 Exception calling "DownloadData" with "1" argument(s): "The underlying connection was closed: An unexpected error
occurred on a send."
At line:1 char:1
+ $bytes = $webclient.DownloadData($uri); New-App -mailbox $Mbx.LegacyE ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : WebException

The EV server used in the commands requires TLS 1.2, and the exchange server powershell / .NET version does not support TLS 1.2, by default.

Use Method #2 above, but first run the following line to enable TLS 1.2:

[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12

Error Message