in fact there is a strange behaviour of Oracle which does not release temp segments when needed and instead of that tells you that you cannot extend TEMP etc ... while it should just free space :/
well nevertheless, my post is to give you a workaround to free TEMP without shutting down your instance, just follow these 3 steps :

alter tablespace TEMP default storage (pctincrease 1);
alter tablespace TEMP default storage (pctincrease 0);
alter tablespace TEMP coalesce;