Excel has been doing strange things all day, to the extent that I just uninstalled and reinstalled Office 365. That didn't help a lot, with the possible exception of reducing the frequency of freezes, and I continue to get an error at a simple line of code to change the interior color of a range defined using the CELLS method. I can copy the line to the Immediate window, replace one variable with the actual number, and run it successfully.
Solved by D. D. in 13 mins