Cannot evaluate communication tasks with at least 10 user processes

Description:
The system cannot evaluate submissions for communication tasks which are configured to have at least $10$ user processes.
The submissions are "stuck" in the evaluation phase and are re-evaluated (indefinitely?).
In the sandbox logs, the status is XX, and the message is execve("./task"):Resource Temporarily Unavailable.
I believe the reason for this behavior is the definition of box_id in cms/grading/Sandbox.py, Lines 861-873.
The code there allocates $10$ ids per worker shard, and when the worker tries to evaluate a submission with at least $10$ user processes (plus an additional manager process), duplicate ids are used.

As a workaround, one could increase the number of ids allocated for each worker shard in Sandbox.py, but perhaps we can find a more generic fix.

Expected: The submissions should be evaluated correctly.

Actual: The submissions are re-evaluated due to sandbox errors.

System Information

CMS version: 1.4.rc1
Was CMS installed: yes
Using a virtualenv: no

Start in cms/grading/Sandbox.py at lines 861-873 and inspect how box_id values are allocated per worker shard. Reproduce evaluation of a communication task configured with at least 10 user processes, then verify that submissions complete without sandbox errors or indefinite re-evaluation.