Twitter has opened access to Analytics for all users. However when you use an Ad Blocker, like Adblock Plus, you’ll get the following error:

For Adblock Plus you can fix this by adding a filter: go to Filter Preferences and on the "Custom filters" tab add a new filter within a filter group (or create a filter group for this rule).

Use this as a filter rule:

@@||ads.twitter.com/stylesheets/ads-allow.css

And don’t forget to Enable the rule!

EDIT: Twitter user @Ertraeglichkeit mentioned a different method:

To understand how this works it’s good to know how adblocker detection roughly works, see here for a good explanation.

To implement this in AdBlock Plus use the following filter rule:

analytics.twitter.com###BlockedAdsWarning

Or install the "Element Hiding Helper" for AdBlock Plus then open the Twitter Analytics Page and choose the "Select an element to hide" option:

Move the mouse around until the red rectangle is around the complete page (because that’s what’s being hidden with the overlay:

In the bottom left corner you can see the html element that is selected. Press the S key to select the object and the rule is composed: