export docx to pdf with powershell script in GitLab pipeline

I have a powershell script to create a pdf from a docx. I can call it manually an it works fine:

param(
    [string]$File,  # Path to document
    [string]$DestinationFolder,  # Path of folder
    [string]$NewName   # New name for converted file
)



# ======== check if parameter are valid ========
    if (-not $File) {
        Write-Error "Parameter '-File' must not be empty"
        exit 1
    }

    if (-not $DestinationFolder) {
        $DestinationFolder = Split-Path -Path $File
    }

    if (-not $NewName) {
        $NewName = [System.IO.Path]::GetFileNameWithoutExtension($File) + ".pdf"
    }



# ======== create destination folder if not exists ========
if (-not (Test-Path -Path $DestinationFolder)) {
    $null = New-Item -Path $DestinationFolder -ItemType Directory -Force
    Write-Output "Path did not exist. Created folder: $DestinationFolder"
}



# ======== export pdf ========
    Write-Output "Export pdf"
    $Word = New-Object -ComObject 'Word.Application'
    $Document = $Word.Documents.Open($File)
    $NewFilePath = [string](Join-Path $DestinationFolder $NewName)
    $Document.SaveAs($NewFilePath, 17)
    $Document.Close()
    $Word.Quit()
    Write-Output "Stored to $NewFilePath"

but if I call it from a GitLab pipeline, I get the following exception at the line $Document.SaveAs($NewFilePath, 17):

At D:\GitLab-Runner\builds\yxFfC2Egu\0\SPS_Entwicklung\test\ci-dev-test\ci\release\docx_to_pdf.ps1
:44 char:5
+     $Document.SaveAs($NewFilePath, 17)
+     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull
 
You cannot call a method on a null-valued expression.
At D:\GitLab-Runner\builds\yxFfC2Egu\0\SPS_Entwicklung\test\ci-dev-test\ci\release\docx_to_pdf.ps1
:45 char:5
+     $Document.Close()
+     ~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull

The script does not finish, but word takes more and more memory. I can only stop it by ending the task with the task manager.

• The gitlab runner service is started with an admin user
• I tried to execute the script manually with the same user, and it worked fine
• I tried the same thing with python and had a similar error ValueError('NULL COM pointer access',), tried this -> same error