Does the Project Template used to create the project have a File Type Configuration that has the Microsoft Word 97-2003 file type enabled? Also can you confirm that these are actually valid Word documents by opening them in Microsoft Word?

Hi Nick Date

Where can I check the  File Type Configuration? but for the second question, yes, they are valid Word documents. 

Hi Mattia Del Franco,

1. Log into the Trados Enterprise web UI, and go to Resources -> Project Templates.

2. Locate the Project Template you are using to create projects in the API, and click on it. The name of the File Type Configuration will appear in the first page.

3. Make a note of the name of the File Type Configuration being used, and then click Cancel.

4. Go to Resources -> File Type Configurations and search for the name of the File Type Configuration from step 3.

5. Click on the File Type Configuration, and make sure that Microsoft Word 2007-2019 is enabled like so:

6. Click Save to save the changes.

If this doesn't work for some reason, then please tell me your tenant number and Project Template name so I can look at this for you.

Hi Nick Date,

Thank you for your answer. I've checked them and all formats are enabled. If you can take a look at it I'd be grateful. 
What do you mean with "tenant number"  and where can I found it? 

Hi Mattia Del Franco,

It is a six digit number that identifies your tenant. You can find it in the address bar of your browser when logged into the web UI. I have highlighted it below for our test tenant:

Please let me know what yours is.