Cash dividends on stock in your 401(k), paid to you, are distributions from the 401(k) reportable on Form 1099-R and are taxed as ordinary income. The stock is owned by 401(k) account, not by you personally, so the dividend is payable to the 401(k), not to you, and you only receive the money by distribution from the 401(k). They are not taxable as capital gains. Why you would have received Form 1099-DIV in the past for such distribution is a mystery, perhaps payer error.
