New installation getting popup “Sorry, Ubuntu 15.10 has experienced an internal error”. I have installed unity and reset as advised. Still have the problem.
What to do - what to do?
Ubuntu 15.10? Unity? Are you in the right forum? 
Yeah - My ubuntu-mate is based on ubuntu 15.04. That’s what pops up when I hover over a mate menu icon in my panel. I was counseled to install unity to fix the problem - didn’t work - uninstalled unity
“Sorry, Ubuntu 15.10 has experienced an internal error” is a pretty genetic message. Usually because of a program or background service that crashed.
apport is responsive for showing this message. It’s designed to easily create and send bug reports so that the issue can be fixed.
I’m still in 14.04.3 Mate, and this story of “Ubuntu had an error …” is going on for a too long time, IMHO … 
Disable or reset Apport.