I have 2 different piwik installations and this morning I upgraded them to 2.8.0 and now both of them show me this error when I access "Visitors / Custom Variables" :
Action 'index' not found in the module 'CustomVariables'.
I've seen another issue about this error but it was for 2.6.1 and the solution didn't work for me (and I'm not sure the problem was the same).
I don't know if it's time to cry or laugh. Since 2.4.1 there was no update without any problems.
I've tried clearing the cache, restarting apache and changing "index" inside Menu.php, but none of them worked, I still get the same error.
It looks like some files were not copied correctly. I'd suggest to copy the files over again to make sure all files are up to date see http://piwik.org/docs/update/#the-manual-three-step-update
You probably won't have to do any database upgrade in the end. Only download latest Piwik from our website, backup config.ini.php and copy all files to your server. You do not have to update the config folder.