Shares of Groupon Inc. GRPN dropped more than 1% in aftermarket Wednesday after the deals and coupons company said it was “facing serious challenges” to remain afloat and offered mixed quarterly results. The company also kept a going-concern warning in a filing and the mention that it continues to evaluate “several different strategies” to improve liquidity. Groupon embarked on a restructuring plan in January and got a new chief executive in March. In the first quarter, the company lost $29 million, or 95 cents a share, compared with a loss of $34.4 million, or $1.17 a share, in the year-ago period. Adjusted for one-time…