7 month puppy suddenly barking at night for no reason

Hello, I have a 7 month old puppy (got her at 3,5 months) that is most of the time great, but at night she's driving me crazy.

We have 2 training sessions per day, some play, and once per day (we tried at lunchtime and switched to 6pm to see if it changes something) we go to the forest for 1h where she's unleashed. She eats when we go home. When it's time to sleep (10:30pm), quick potty break, clean paws, lick mat, and she sleeps with us in the bedroom. We turn off the light directly and put white noise on the phone.

The thing is, she starts barking after 5 min, even when she's not done with the licking mat. She didn’t do that before, it started 1 month ago. She's barking at the door with ears up and a tense body, so I suppose she's afraid of something. She's not barking to have my attention.

She doesn't bark like that during the day.

I don't really know what to do. We tried more and less exercise, sleeping in a different room, letting her free roam (she's not destructive and crate is not a thing in my country), but it doesn't change. If we ignore it, it continues. Sometimes she will even scratch the door. If we get up, open the door, and show her there is nothing, it stops, but she starts again a few minutes later. If I just say "it's okay, thanks," it doesn't change anything. Best case, it stops after 10 min of nonstop barking. Worst case, 1h+.

Once she's asleep it's okay and she sleeps the whole night. She has no problem sleeping during the day.

Any ideas on what could be causing this or what we could try?