I started to make the changes as suggested on the link, but it didn’t make any difference.
I decided, instead, to copy the AdafruitIO_WiFi io(IO_USERNAME, IO_KEY, WIFI_SSID, WIFI_PASS); line from the config.h file into the main.ino file, and that seemed to fix it…
(…other than the fact it’s now complaining that IO_USERNAME, IO_KEY, WIFI_SSID, WIFI_PASS are all undefined.)
This is making me think that the editor is ignoring the code inside the config.h file (which is being included right at the top via #include "config.h"), something it wasn’t doing before.
This is my main.ino file as it exists right this second, along with my config.h file.
Thanks.