Wordpress MU is a feature in which we can run multiple wordpress sites using only a single wordpress installation. That means, the users, themes, plugins will be common, but the contents will be different for different sites.
Check whether your wordpress installation have correct file and folder permission for proper functioning. Improper file permission may lead to security threats also.
In this tutorial we shall share all the commands necessary to host multiple websites in a VPS server. We used VPS servers from Linode and Digital Ocean. Both are excellent and best value for money.
In case, your MyBB ACP -> Configuration -> Server and Optimization Options -> Enable search engine friendly URLs? is set to DISABLED then use the following robots.txt.
Any image when uploaded into wordpress, it will automatically creates few more instances of that image of different size based on media settings. Recommended media settings is shown below: