Completing seemingly simple quest objectives in Kingdom Come: Deliverance 2 can be tricky. This guide helps you navigate the "Wedding Crashers" main quest, specifically locating the newlyweds for your congratulations.
After realizing Otto von Bergow won't attend Lord Semine's wedding, the quest shifts focus. To complete "Wedding Crashers," you must congratulate the newlyweds before leaving the festivities. Finding Agnes and Lord Semine, however, proves challenging.
Complete any other wedding activities before speaking to Agnes. This conversation concludes the quest and takes you to Trosky Castle. Most optional wedding content occurs before the ceremony.
While you can ask other wedding guests about the newlyweds' whereabouts, they won't be helpful. Instead, proceed directly to Agnes' location. If you previously pilfered schnapps from the wine cellar, finding her will be easier.

Enter the main building. Across from the group conversing near the stairs, you'll find a guard by a door leading to the wine cellar. Agnes is inside, alone and distraught, indicating the wedding hasn't gone as planned.

When you speak with Agnes, choose the option to congratulate the newlyweds. You'll discover Olda, her husband, has left without explanation, a recurring pattern in their relationship. Your choices here might influence later quests, potentially requiring you to side with Semine or Hashek.
Regardless of your conversation with Agnes, a brawl erupts in the cellar. You can either wait it out or participate. The quest outcome remains the same – Henry and Hans are imprisoned at Trosky Castle.
This concludes the "Wedding Crashers" quest. Your next main quest, "For Whom the Bell Tolls," requires timely task completion to gain reputation.