Back on March 12, 2019, with TTax 2018, I spent over an hour on the phone with 2 support agents, and like you, the only solution was to start over from scratch, since that form cannot be deleted.
I do believe the internal error is actually on the indicated form, since when it's been on other forms, deleting that form and recreating it has fixed the problem.
THIS IS FOR INTUIT IF YOU ARE LISTENING, coming from a retired computer scientist. I imagine the "estimated" attribute exists for many internal variables as well as user entered variables, and that something is writing out of bounds and stepping on the estimated attribute of some internal variable that happens to be adjacent to it on the stack. The out of bound access could be as simple as writing an int into a byte variable address. I know from experience that this can be quite hard to find, but with the right tools, it can be done.
Please hire someone to find this and fix it. It's been going on too long!!!!
