Go to "In the Milling Room" at night in the eastern area of Kleinstadt and wait there until some ghosts appear.
Go to Heinz Mueller and ASK HEINZ GHOSTS. ASK HEINZ CURSE.
Go back to the mill and wait there until Heinz comes in and steals some grain.
If Heinz is really far from the mill or is taking too long to get there, you can move him closer by typing ASK HEINZ THIEF.
Heinz will not steal things if there are other mobs in the room. You must be alone. DO NOT KILL ARMINDA (though she shouldn't be in there).
After Heinz steals grain and leaves, GET FABRIC NAIL.
Find Heinz and GIVE FABRIC HEINZ. ASK HEINZ THIEF, ASK HEINZ BLACKMAIL, ASK HEINZ ARMINDA, ASK HEINZ BRIBE, and ASK HEINZ YES.
Go to Arminda and ASK ARMINDA GHOSTS, ASK ARMINDA HUSBAND, and ASK ARMINDA HUSBAND IS A THIEF.
Follow Arminda and she will lead you to Heinz. If she does not start yelling at Heinz, you can repeat the previous set of questions.
NOTE: It is possible to skip the blackmail/bribe portion of the quest by taking the fabric directly to Arminda. If you want the double-crossed whois, make sure you talk to Heinz first and get him to pay you off.