iCloud keeps signing out since El Capitan update. Why?

Since updating my Macbook Pro early 2011 to OS El Capitan it has been a lot faster in opening apps and logging into it, however there seems to be a problem with iCloud sign-in. If I go to send an email from mail via my gmail account I get the following message 'This Mac can’t connect to iCloud because of a problem with “*********@gmail.com'. I open iCloud preferences and sign in again but it will shortly sign me out again. 😟

I recently upgraded my memory from 8gb to 16gb but I can't see this being the problem, unless I'm wrong as not that technical when it comes to hardware!

Anyone else get this problem, if so is there a solution as all I can find on here is something relating to kaspersky and I don't have that installed.

Thanks