[moved from Creating PDFs to Acrobat Reader]
Hi Joostb,
Sorry for the delay in response.
Firstly, check if there are any pending updates for Reader, to check for update, launch Reader>Help>Check for updates. Try restarting if you installed any updates. Then try to check if you have email client listed in Reader Preferences. To check Launch Acrobat Reader>Edit>Preferences>Email Account, you may set the one which you want if you can see in the list.
You may also want to take a look of this if you want to remove extra steps of new UI: https://helpx.adobe.com/in/acrobat/kb/send-email-attachment.html
Let us know if this works for you.
-Tariq Dar