I find that Element Hiding Helper for Adblock Plus 1.2.3 appears to cause the dreaded "Extension data is still loading ... try again in a few seconds" error.
Almost every time it's enabled I get the error. Every time when it's disabled, FEBE works fine with no other changes.
Note: This only happens when the Backup userChrome files option is selected in the What to Backup->Backup type tab.