PDF-Datei mittels Button an eine E-Mail anfügen

MENU - PREFERENCES - Email Accounts - make sure these are unchecked. 

@silvia_hetzer The older version of Outlook was a desktop application that had deeper integration capabilities with other software like Adobe Acrobat. The new Outlook version, being web-based, has limited integration with local desktop applications. This isn't a mistake you're making, but rather a technical problem stemming from the way the two applications interact. There is a known compatibility issue between Adobe Acrobat and the new version of Outlook.

Because of this, you could try and is the simplest solution is to save the completed form to your computer first. Then, open a new email in Outlook and manually attach the form by navigating to the saved file. Because I haven't updated my Outlook (this is why I am not a big fan Microsoft!), instead of using the button you created, try using Adobe's built-in "Send File" feature. This is typically found under the "Share" menu. It will often open a new email in your default email client and attach the file. While this might not work with the new Outlook either, it's worth a try.

Of course, if the functionality is critical for your workflow, you might consider reverting to the classic version of Outlook. Maybe there is an option available in the settings of the new Outlook. You can usually find a toggle or a button that says "Revert to classic Outlook" or similar.