open .dotm and save as .docx with filename from bookmark

Try:

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Note: This code requires a reference to Word, via Tools|References, but I assume you already have that since you're already using Word constants. Templates should not ordinarily be opened; rather, you should use the .Add method to create a new document from them. The code will still fail, however, if the "Aktbezeichnung" bookmark is empty - you don't have any code to update it - or contains characters that are illegal for a filename.