Cannot enable issue notifications

Summary

On gitlab.com, I cannot enable issue notifications: the slider "Notifications" is gray and hovering it turns the cursor into a "no go" sign. No tooltip or other indication for why this is is given.

Steps to reproduce

• Open any issue. (I tried #235138 (closed), #331508 (closed), CardBook/CardBook#1259 (closed))
• Try to enable notifications for that issue.

I am doing this with Firefox on Linux.

What is the current bug behavior?

The slider to do this is disabled.

What is the expected correct behavior?

I should be able to use that slider to enable notifications for an issue. At the very least, an indication should be given for why the slider is disabled. (Disabled UI elements without explanation are frustrating.)

I do receive email notifications for my own actions, such as creating this issue. So notifications work fine, I just cannot turn them on for existing issues.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com.