Exactly that same error is seen in this post (Dashboards widgets missing since emonpi update - #3 by Frits)
We ended up having to reset the Module repo. Looks like something may be broken with the update (@glyn.hudson, @TrystanLea, @Gwil)
If you have SSH access;
cd /var/www/emoncms/Modules/dashboard
sudo git status
please and from the admin screen look and see what the version number of the Dashboard Module is.
[edit] Can you do a database check and update from the administration page as well please.