UPDATE: There is a fix for this. Once your system has 184.108.40.206 applied it will be resolved.
Have you been seeing popups wherever you navigate inside of CRM in the last few weeks (as of November 9, 2017)?
If so you are not alone.
There is an internal bug that is generating error messages as follows, it is not your customization code.
Failed to execute ‘postMessage’ on ‘Window’: 2 arguments required, but only 1 present
<Message>Argument not optional</Message> <Line>8</Line><URL>/_static/_common/scripts/cdnavailabilityworker.js?ver=-896173260</URL>
There is fix coming out for this issue in the next few weeks. It will be in release 220.127.116.11 for North America.
Keep on the watch for it.
In the interim, you may want to change the privacy settings to automatically send an error report to Microsoft without asking the user for permission.
Go to Settings, Administration, Privacy Settings, check the appropriate box.