How are you opening the .sch file? By clicking on the file icon? Do you have .sch (which is a common suffix) associated in Windows with another ECAD program, which is the one complaining it doesn’t understand KiCad schematic format?
What if you open from the 5.1 project manager?
Mystery is solved, sort of. Before starting work today I tried to recall anything that I might have done over the last year to this computer other than routine OS updates, and there was one thing that came to mind. Several months ago I was running out of space on my 1 TB hard drive and installed a 2 TB M.2 PCIe NVMe SSD to double my space and improve speed. My W10 is installed on a 128GB SSD which is C: while the data/app drive is D:. After installing the SSD, I copied everything from the old hard drive to the new SSD, and mapped the new SSD drive as D:, while keeping the old drive intact and mapping it as G:. I can’t recall if I simply did a straight copy or used an image copy utility.
So this morning I first tried accessing the old 5.1.12 projects on the old hard drive. Same problem with the missing “=” errors. Next instead of running the old KiCad.exe from the W10 start menu, I ran the KiCad.exe from the old hard drive. Bingo! Schematics pull up fine, whether loading the project from the old hard drive or the new SSD. Next I uninstalled KiCad 5.1.12 using control panel and reinstalled it from my setup archive (yesterday I reinstalled 5.1.12 but simply overwrote the files). Now everything is working. I’m getting a few library warnings probably because I saved some custom symbols in the regular library instead of my own when I uninstalled 5.1.12 but these will be easy to fix.
I’m not going any further with this to understand why this causes the errors, but the what caused it part is solved. I have not noticed any other problems with data or apps since I installed the new SSD. Word processing, spreadsheets, code development tools, etc. all seem to work although I have not tried everything since I installed the SSD. I will maintain the original hard drive just in case I run into anything else odd. Thanks again for the assistance and I’ll let you speculate what might have happened but this is above my pay grade…