After a power outage, my FreedomBox Pioneer Edition (Olimex A20-OLinuXino-LIME2) board shines red next to the power barrel jack and my router doesn't find it.
I remember it used to shine green when working.
What does the red light mean? Is it sign of a Hw problem or could it just be that the SD card got corrupted?
The red LED being on is good thing! It is power LED. It is indication that the unit is successfully powered.
It is not an user LED and it is not controlled by the software.
Try with another card. Freedombox images can be found here:
https://ftp.freedombox.org/pub/freedombox/pioneer/
You can try with the stable image, or maybe go inside the "weekly" sub-folder and try the latest image.