Dog all of a sudden afraid of the dark

I have a 5 year old German Shepard/Terrier/Husky mix. As of recent she has been refusing to go out in the yard at night time to go potty before bed. It feels really sudden and out of character for her because she loves going outside during the daytime.

I also have another dog who goes out at night and day no problems. The two of them go out together and play during the day with no issues.

So far I’ve tried to walk out with her, use a flashlight to provide her additional lighting – but she refuses to go beyond the back porch area. I’ve tried treats and praise whenever she’s outside but all she will do is sit by the back door and cry to go inside.

I have no idea if something has spooked her to the point where she doesn’t want to go out at night, so I am not sure how to rebuild her confidence surrounding it.

I am concerned because this means she’s holding her pee a minimum of 12 hours overnight now that it gets dark so much sooner outside. I have expressed this concern to the vet and they think it’s 100% behavioral as she has a clean bill of health.

Any advice is appreciated before I reach out to a behavioral trainer!