I wonder what the issue is? Unfortunately until I can reproduce it, I'm only guessing... In the meantime, here are some questions to answer that might assist me to work out what the issue might be:
Are you using a US international (or compatible) keyboard? If not, could you please retry using a more simple password (e.g. just use characters that should map 1 to 1 with a US international keyboard)?
Do you use a non-English locale on your PC (that you use to access Proxmox via web browser)? What about Proxmox itself? (In theory that shouldn't matter, but perhaps it might?)
When you log into your container, are you doing that from a "proper" SSH client (e.g. OpenSSH)? Or are you using the Proxmox in browser terminal? E.g. the 'NoVNC' in browser terminal?
Have you also tested logging in via Webmin and/or Webshell? Do either of those work with the password you set?
Must be something weird going on - but I can't reproduce?
I wonder what the issue is? Unfortunately until I can reproduce it, I'm only guessing... In the meantime, here are some questions to answer that might assist me to work out what the issue might be:
Are you using a US international (or compatible) keyboard? If not, could you please retry using a more simple password (e.g. just use characters that should map 1 to 1 with a US international keyboard)?
Do you use a non-English locale on your PC (that you use to access Proxmox via web browser)? What about Proxmox itself? (In theory that shouldn't matter, but perhaps it might?)
When you log into your container, are you doing that from a "proper" SSH client (e.g. OpenSSH)? Or are you using the Proxmox in browser terminal? E.g. the 'NoVNC' in browser terminal?
Have you also tested logging in via Webmin and/or Webshell? Do either of those work with the password you set?