I neglected to mention that I can work around the problem by using Cmd-R. In other words:

1. Click the magnifying glass.

2. Type "1335" into the search field.

3. Choose Estimate 1335 from the drop-down candidate list.

4. The spinning progress meter will spin for at least two hours (that's the longest I've let it go so far).

5. Type Cmd-R.

6. Estimate 1335 opens immediately.

I think that implies that it's not a sales form error. (I can also enter into evidence that I haven't modified any of my forms in the past several months.)

If I execute the steps above in an incognito browser window, I don't get the problem at step 4. 

After clearing my browser cache, the form opens immediately (without even showing the page that has the rotating progress meter on it). Problem solved.

Thank you!