I wanted to add an "Update Records" action in a process and set a new checkbox field to true which I created just before. However, even after reloading process builder the field is not available in the list of fields to update... is the list cached somewhere or am I missing something obvious?
I tried this and saw my checkbox field no problem, so I'm going to hazard a guess that you have fallen into the "Select a Field" overlay trap. Which is this:
When you click on the "Object" field and it gives you the "Select a Field" overlay, it's REALLY asking you to select an object, not a field. So it gives you a list of lookup fields to navigate to a related object.
If you click on the object name, and click save, it will then give you a list of fields on that object that you can update.
I have had a problem with editing a process where the field was not showing in the process builder.
The problem was because the process was created whilst the field was 'not required', then the field was made 'required'. The process needed updating to set the value which then wasn't available.
The solution was to make the field 'not required', update the process, then set the field back to required.
btw: if the field is required and a new process created to create/update an object then the field is pre-loaded into the form and a value must be given.