Cron is trying (and failing) to open env file: /etc/environment

Answering all of your questions

1. Why is cron/pam_env/pam_unix trying to open that file in the first place?

See BUG #646015. In some cases (like locale related stuff) this file is deprecated. But it is still used system-wide, and log is made whenever it is missing.

2. If they legitimately expect it, why isn't it there?

Cause maybe the bug isn't fixed after all. Steve Langasek (BUG #646015) said it is, and new systems should create that file using postinst scripts the same way old systems being upgraded should already have that file.

3. What should I do about this?

• Run dpkg-reconfigure libpam-modules and see if it will create the file through its postinst script.
• If that does not work, create the file manually with touch /etc/environment

It's also interesting to report your issue to the Devuan Project with details of the problem and your setup since this issue was resolved before the Debian/Devuan fork happened.

An inelegant but perfectly valid solution is to give those modules what they want: As root, execute

touch /etc/environment

and to make sure the permissions are right, just in case:

chmod 644 /etc/environment