Looker Studio calculated fields not working

Hi everyone, around a week ago the calculated field functionality stopped working in the Explorer view of a data source.

I can still click on Add Dimension, Add Calculated Field, and see the New Field dialogue box. But here you can enter a new name for the field but the formula area is greyed out with no ability to input a formula.

I've attached a screenshot - could it be related to the orange triangle that is showing next to my data source? I think that is also newly appeared.Calc field.PNG

Does anyone else have the same issue and/or a solution. Many thanks in advance.