(Thanks to Vchat20 and those who contributed at the phpbb.com support forum for the following information.)

If your username is 2 characters or below, you cannot search for posts you've made in the past. For technical information, read this.

Please note that I am not willing to fix this problem manually, as the risks involved in directly editing the phpBB software coding outweighs the issue at hand. You can fix this for yourself, however, by making your username longer. (This can be done via your profile editing control panel.)