Error " EXP 81 GOT A1 AT 236653 " during install of APM 2.6 version 3.2.1

Noobie here - 1st post

Whirlwing 550-type hexacopter with a Turnigy TG-9X transmitter, Turnigy receiver, Arducopter Flight Controller, CJMCU-GPS, Turnigy 28-265 1200 kv brushless motors, Turnigy 20A Plush ESC’s.

When I try to flash the APM 2.6 version 3.2.1 firmware to the FV via Mission Planner the download from the internet appears to work fine, but the verification check which automatically takes place ends with "Download successful. EXP 81 GOT A1 AT 236653 " and it gives me a clickbox which says “OK”. Clicking on that sends me back to the screen which starts the firmware download all over again. It never lets me proceed to the next screen, so any further activity is not allowed. (I’ve tried this down numerous times in the last couple of days but get the same results every time.)

This 'copter was running APM 2.6 previously, but I cannot seem to find anywhere on the internet where I can download that version now.

A. Does anyone know what “EXP 81 GOT A1 AT 236653” means? I’m suspecting it is saying “Expected HEX 81 but got HEX A1 at line 236653”, but that doesn’t mean anything to me at all.

B. Does anyone know of a website where I can download APM 2.6?