Is this happening on a mobile browser or the Gmail Android app? Any specific filetypes? Have you tried saving the attachment in Drive and then open/download from there?
There is some incompatibility with CloudFlare's 1.1.1.1 DNS app. Disabling it was an instant fix for me.
Credit to Stephen Ross: https://support.google.com/mail/thread/4280094?msgid=6847305
There are many threads about this issue, but none were created before April, when CloudFlare announced its VPN ambitions and the 1.1.1.1 app gained a tremendous amount of traction.