[SalesForce] Check if lookup field is not null in Process Builder

You can't fire from a formula change, but there is strange path. Have a Workflow rules fire on EVERY change (every edit and where CreatedDate not blank). And have this do a field update of the formula field into text field (or number or date, whatever matches). On the field update check the box that it will re-fire workflow evaluations. Now you should be able to do your Process Builder (or other Workflow) off of the text field. Bit convoluted I'll admit, but it should work. Good luck.

I ran into this problem in our org. Move the lines of criteria about the "Account ID is null - false" & "Contact ID is null - false" to the first two lines of criteria. I'm not sure the reasoning behind this. We contacted support and had a similar error and couldn't figure it out. When we put the boolean criteria in the first two lines as they suggest, the Process Builder worked with no errors.