Bug Custom Rank tool giving errors when adding tiers and ranks

When attempting to add new custom ranks and custom tiers to packs, e.g. as part of the forming process, I am encountering the following error:

Most of the time, the change goes through despite the error, but sometimes not. Sometimes the change goes through multiple times when the error happens. Refreshing does not clear the error, it can only be bypassed by pressing Back on the browser. I'm not able to identify what's different between when it does work and when it doesn't, but either way, it's making it difficult to add and change custom ranks.

Browser: Google Chrome Version 96.0.4664.110
Affected: This issue is universally affecting every pack's custom ranks.