Error establishing a database connection

Hi Guys!
no idea if the below message is an issue of the domain mapping plugin but it frustrates us a bit

Error establishing a database connection

normally a user goes and registers for a site/blog and once activates his account then he can access his site via http://sub.multisite.com and via http://sub.multisite.com/wp-admin.php

Also when i try to do a network update i get the below error

any advise on how to sort this?

last 3 hours we got 300 signuos and the clients cant access their sites?

PS. we are using the Blogs MU theme and all plugins and themes are from wpmudev

Warning! Problem updating . Your server may not be able to connect to sites running on it. Error message: A valid URL was not provided.