Clicking "Lock and Edit" throws Item archive error has occured

Recently i have been baffled to figure out why clicking "lock and edit" throws an "Item Archive error has occured: Object reference not set to an instance of an object"

The problem is because of the Security setup. Now the Sitecore Tree setup which we inherit from another company is quite messy. Here is the structure


Sitecore
--> Content
--> Home (aka main website)
--> Page 1
--> Mini Website
--> Page 1
--> Another Mini Website

We have a requirement whereby a user let's called him user1 is allowed only to look at "Mini Website". Of course the approach is to use default Sitecore Security feature. Ideally when user1 is logged in the Sitecore tree should look like below

Sitecore
--> Content
--> Mini Website
--> Page 1

That's the cause of the error. Basically Sitecore->Content->Home need to be available.

So the Tree setup turn out to be


Sitecore
--> Content
--> Home
--> Mini Website
--> Page 1