Thanks to @山楂树 's information, this ship's only hatch is proved disabled due to changes on fairing logic in KSP 1.8. That lander can is now "enclosed" inside the fairing.

This means Kerbals in pressurized cabin won't be able to go EVA anymore.

At the moment I have no good solution without unwanted change of design. For surface operations, have some Kerbals sit on the command chairs before departure.
--------------------------------------
感谢 @山楂树 指出，本飞船唯一的舱门现已失效。原因是 KSP 1.8 改变了整流罩逻辑，游戏判定该着陆舱已被整流罩完全遮蔽。

这意味着加压座舱内的小绿人无法出舱。

目前我想不到合适的解决方案，除非大改设计。要进行地面活动，请在起飞之前将小绿人安排在指令座椅上。