Why can't I edit any of my Project Custom Fields and general Project fields seem to be greyed out?

If you encounter this scenario then you probably are not the first person that is currently looking into this Project at the very same time. 

When you want to access any Project and you get this message: 

..then another User, in this case Peter, is already inside of this Project and will only allow you to access you in Read-Only-Mode.

Your Edit Project window will look like this with some fields greyed out: 

In this Project Read-Only-Mode you can and can't do the following: 

In Read-Only mode you cannot
- change any of the standard fields in the top left corner of the Project Window
- change any of the Custom fields
- modify the Budget
- add Extras (Services, Material, Expenses and Objects) directly from the Project Window
- change rates of Extras that have been added directly to the project within the Project window
- change View settings
- change Project Status
- select another custom template > menu icon for templates is hidden
- do various types of actions from the Eye menu as many items are hidden
- create Media Orders, Virtual Bookings, Work Orders

You can
- change the Rate on a financial line in a Booking, on both booked Objects and Extras
- edit a Booking > click on clock icon and select Edit Booking
- enter Budget window to view only