Writing this as info for anyone who have the same problem I had:

Problem: I couldn't view nor download email attachments.
Reason: Insecure Content for the site mail.zoho.com was blocked in my browser's settings.
Solution: Allowing Insecure Content for the site mail.zoho.com in my browser's settings fixed the problem.

Background info:
- I don't use 2 step authentication
- the browser I use is Chromium 83.0