Hello Stelios369, I do agree, that it is corking. Have seen that in some of my cacti and euphorbias. Sometimes environmental factors influence corking, like if the Euphorbia was still dormant and got too wet and humid, it will do that. But once it has been kept dry and warm, that damaged part will slowly dry out, harden, and heal. If it was facing the sun as it dries out, all the more it tans.
Glad to see it is corking, so it has healed and recovered. The blemish stays, I just call it their battle wounds. My Euphorbias are also awake now from its slumber, active growth ensuring.
If you intend to repot, heed the advice to handle cautiouslly, since it is a toxic plant.
I find my Euphorbias love warm to hot temps, gets quite thirsty, so I can give them more water but got to keep the media very well draining, making it gritty helps protect the base part of the plant from moisture damage.
