I would like to report that I was able to successfully re-flash my inverter and get it working again. The short answer is trying to flash it with a Windows 10 computer failed and when I switched back to my Windows 7 laptop and it worked fine. I was even able to successfully revive an Inverter that appeared to be bricked. Here is the whole story.
After the first re-flash attempt, the Inverter still worked but it would not see the solar (or charge the batteries) at all. The MPPT was completely disabled.
I contacted Growatt and they sent me a newer ISP tool V5.03 and said that should fix it.
The second re-flash attempt failed just like the first but this time the inverter would not even come back on.
It just set there with screen off and the lights flashing like this
No attempt to reset or restart it was successful. Now nothing worked, I basically had a brick with flashing lights.
I contacted Growatt again and they told me that even in that condition, if I started the flash process first and then turned the inverter on, it should work so I tried it a few more times and.... NOTHING.
Then I remembered reading something on this Forum about people having problems with Windows 10 but Windows 7 worked fine. I recalled, when I first got this inverter, I was able to upgrade the firmware just fine AND it was with a Windows 7 laptop so I decided to give it one more try.
I dug out an old Windows 7 laptop and installed the original USB driver on it and using the old ISP Tool V4.30 and the old firmware I used 2 years ago when I first got the unit, I did the following.
With Utility AC power still attached to the Inverter, I removed battery power from the inverter and let it die.
I hooked up the USB cable and opened the ISP Tool V4.30.
I selected the COM3, choose the Firmware to flash, opened communications to the inverter, (the time and date popped up on the screen showing me I was successfully communicating with the inverter) and hit the FLASH button.
The Send: Start! command began scrolling in the left window.
With the power switch on the inverter set to ON, I immediately went and added battery power (soft start through a resister) to turn it on.
After a few seconds for the capacitors to charge up, I switched the battery disconnect to ON.
Progress on the ISP Tool screen started immediately and it went through the re-flash process just as it is supposed to.
It Erased, Flashed the new firmware, Verified and once complete, the inverter came back on.
The only problem was, it still did not recognize the solar panels.
The inverter was again working but still no solar charging.
I switched the building back to utility power to take the load off.
I left the inverter on and the solar connected.
I used the new ISP Tool V5.03 and successfully flashed the newest firmware with zero problems.
When the inverter came back on this time, everything (including the solar) is working again as it should.
I will have to wait to see if the original problem (intermittent solar charging failure) is fixed or not but at least it is back working again.
It appears the problem has something to do with Windows 10. I must have tried a total of 10 times with the Windows 10 laptop and it never worked, just made things worse. The very first attempt with my windows 7 computer (even to an inverter that seemed to be bricked) worked the first time... perfectly.