Some new users are still able to upload rectangular avatars?

I don't know why it's not working for some users. You'd need the original image they're using to figure out why it specifically is causing problems, but it's likely due to the image having weird aspect ratio like a large landscape image or really large portrait image. It's also possible whatever device they uploaded from isn't loading in the cropping behavior for some reason as again the cropping is client side, which if that fails server side just resizes it to fit within the maximum.

The sizes I recommend for the avatar are as follows.

Maximum Image Height: 160
Maximum Image Width: 160
Maximum Thumbnail Height: 80
Maximum Thumbnail Width: 80

There isn't much reason to have an avatar as big as 610px. The canvas layout won't even display an image larger than 180px.

With CB 3.x I'll be entirely rewriting all core fields including the image field, which will get a significantly more robust cropping functionality. The plan is to store the original (up to a maximum), resized, and thumbnail instead of just resized and thumbnail to allow future edits as well as minimum sizes.

Thank you! I think we set it years ago to 610, so we got better pictures for sharing user profiles on social media like Facebook (og:image) share's. I have now changed it back to your settings, because nowadays, a CB Auto Action generate custom images for all the social shares where the Avatar is only a small part of it. Maybe resetting it to 160 will solve the problems, because for the width it was exactly that 610, so the max values worked I guess

I'm really looking forward to CB 3.x with all those new options. Your avatar-field improvements sound very good. Thank you so much for all your great software development!