we just tried the upgrade from 2.1 to 2.7, but only get a strange dashboard afterwards (please see attached screenshot).
We wiped out the complete webspace und re-uploaded version 2.7, but get the same result.
Deleting the browser cache on FF, Chrome, IE - still not working.
Delete the entire TMP folder within Piwik in order to have it rebuil, because thats where css is stored.. i had some css issues with the dashboard as well (back on other version upgrade) ..dont know if it is the same mechanism but thats how i fixed.
Thanks for the offer. We did look at the javascript files, but were not able (or capable) of finding something. I was so free to sent you the access account. Thanks for your help.