I just downloaded the Encryption module without any issues
Silly mistake on a Friday afternoon – I am using 7.22.0 so the first Encryption module version in the app store wasn't available, hence the lack of download button. I used the All Versions tab and chose the encryption module suited to my modeler version and was able to download. For anyone interested:
A workaround when you can't download from the Mendix client might be to download it from the website here and then manually import it into your project.
Edit: I see you found the solution while I was typing!