The answer
The saying of the Bible " Do not be overly righteous, Nor be overly wise " (Eccl. 7:16), does not mean the person should not grow spiritually and does not mean there is a behaviour higher than the righteousness that God requires from us.
It means that the person behave within his spiritual level without spiritual jumps, otherwise he could be bit by a strike of self-righteousness.
The spiritual person does not "think of himself more highly than he ought to think, but to think soberly" (Rom. 12:3). Don't walk in the way of righteousness over zealously but step by step until you reach. The evil can easily fight with strikes of self-righteousness pushing a person to higher degrees that he spiritually cannot sustain. The person will be unable to continue, then falls into distress and despair. During his short practice in these spiritual levels he might fall into arrogance and judging others. He will murmur
against his spiritual father as if he does not wish perfection for him.
So do not be righteous in your eyes, do not be overly wise, go on slowly and quietly without jumping into levels that you might not be able to continue in, and then might be troubled spiritually.
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Thrice-Blessed Pope Shenouda III of Blessed Memory