"Error reading cookie"

I just starting getting this javascript error in my production app:

Error reading cookie: TypeError: document.getElementById(...) is null

If I turn on Firebug then I see the following console message:

Skipping cookie data sc_is_visitor_unique=rx10137985.1419009621.A48ADBF775D24F0581FA61B45B028F2D.

I am using Firefox 34.0.5
I looked at my Firefox cookies and I do see the statcounter cookie

Clearing the browser cache and rebooting does not help.
Reinstalling the statcounter code does not help.

Deleting the statcounter code makes the problem go away.

This problem does not appear in Chrome or IE8.

Any advice?
Here is a project id showing this problem: 6560496
My statcounter user name is Joseph_E

I tried deleting the cookie sc_is_visitor_unique
and then accessed one of my html pages that has a counter.
That page displayed fine and the cookie I deleted was placed in my browser.

But my production app still shows the problem.
For now, I have deleted all statcounter code from my app.
More investigating has traced this problem to what apears to be a weird timing issue (likely asynchronous) between execution of the statcounter code and a js file that is being loaded.

BTW, none of my code uses cookies.

I thought I had discovered a fix. I was wrong.
Last edited:
OK, this time I did fix the problem.

I deleted the statcounter project that was causing the error and made a new project.

I am using the code for the new statcounter project in my production app and all is fine.