Hi Tamas Meszaros , the problem might be that yr adding credentials directly to the project's credential store, but the DeepL provider needs the API key to be available during provider initialization?

try this:
Here's how to properly configure the DeepL provider using the approach from our working project automation code:

// Create your MemoryResource with DeepL configuration
var memory = new MemoryResource
{
    Uri = new Uri("deepltranslationprovider:///"),
    Credential = "YOUR_DEEPL_API_KEY_HERE", // Use the Credential property, not separate username/password
};

// Then use the AddTranslationProvider method pattern:
private static void AddTranslationProvider(FileBasedProject project, MemoryResource memory)
{
    var provider = GetTranslationProviderReference(memory);
    if (provider == null)
    {
        return;
    }

    var entry = new TranslationProviderCascadeEntry(provider, false, true, false, 0);

    var tpConfig = project.GetTranslationProviderConfiguration();
    
    // Clear existing entries if needed
    tpConfig.Entries.Clear();
    
    tpConfig.Entries.Add(entry);
    project.UpdateTranslationProviderConfiguration(tpConfig);

    // Add credentials AFTER configuring the provider
    if (memory.Uri != null && !string.IsNullOrEmpty(memory.Credential))
    {
        project.Credentials.AddCredential(memory.Uri, memory.Credential);
    }

    UpdateTranslateSettings(project);
    project.Save();
}

private static TranslationProviderReference GetTranslationProviderReference(MemoryResource memory)
{
    TranslationProviderReference provider = null;
    
    if (memory.Uri != null && !string.IsNullOrEmpty(memory.Credential))
    {
        provider = new TranslationProviderReference(memory.Uri, memory.State, true);
    }

    return provider;
}

Complete Working Example for DeepL:

// Configure DeepL provider
var deepLMemory = new MemoryResource
{
    Uri = new Uri("deepltranslationprovider:///"),
    Credential = "YOUR_DEEPL_API_KEY_HERE"
};

// Apply to project
AddTranslationProvider(project, deepLMemory);

// Now run your pretranslate task
var targetFiles = project.GetTargetLanguageFiles(LanguageRegistryApi.Instance.GetLanguage(targetLanguage));
var result = project.RunAutomaticTask(
    targetFiles.GetIds(),
    AutomaticTaskTemplateIds.PreTranslateFiles);

Alternative Approach (if you must add credentials separately):

// Add credential BEFORE configuring the provider
sdlProject.Credentials.AddCredential(new Uri("deepltranslationprovider:///"), APIKEY);

// Then configure your translation provider cascade
var tpConfig = sdlProject.GetTranslationProviderConfiguration();
var provider = new TranslationProviderReference(new Uri("deepltranslationprovider:///"), null, true);
var entry = new TranslationProviderCascadeEntry(provider, false, true, false, 0);
tpConfig.Entries.Add(entry);
sdlProject.UpdateTranslationProviderConfiguration(tpConfig);
sdlProject.Save();

// Now run pretranslate
var result = sdlProject.RunAutomaticTasks(projectFiles.ToArray(), sdlTaskTemplateIds);

Hi Tamas Meszaros , check if you also should also pass state information with the connection when adding the translation provider reference in the cascade entry.  The problem might be related to the API version that is required, associated with your API key.

What I'd do is
1. Add the DeepL provider to your project and choose the correct API version 1 or 2
2. Confirm it works in your project
3. Save and close your project
4. Open the project.sdlproj in an editor and recover the state content from the cascade entry.
5. Pass the state content when adding our translation provider reference to the cascade entry.

It should look something like: "ApiVersion":"V2 (DeepL Pro API subscription)" or V1 if your API key support V1

make sense?

Hi Patrick,

thank you for your help! But I have just realized that I made the problem for myself...

I was tricking with the credential store, because occasionally, I have to reload the project after manipulating the project xml, so I was saving the project credential store object and then restoring with reflection. Which works fine in some cases, but not in this case, because apparently some parts of studio keep referencing the original credential store that remained empty. Without this "hack", the deepl provider works as expected...

thanks again!

Tamas