1)You can have Snapcenter server on LUN too. if you plan to take backup of that and its not mandatory.
2)Please try below steps to resolve the user issue:
1. Login to SnapCenter Host.
2. Open IIS manager
Select Application pools
Select SnapCenter
Advanced settings
Select Identity
Select Custom account.
In custom account give the account which was used to install snapcenter.
3. After changing the customer account to the account which was used to install snapcenter. Restart the SnapCenter site.
4. Clear the Brower cache before login to SC.