Conversational model returns empty string after a while

I've been experinmenting with Huggingface models and I've set up a chatbot with DialoGPT. It works pretty well, but after a while it stops answering and just returns empty strings.

Before this it will start to give shorter and shorter answers. Any idea what can cause such a behavior?

I'm using the medium-sized model with a max_length of 2000 and added a repetition_penalty=1.3, but other than that I didn't change any other parameters. I also add the previous message back to the conversation history so it can keep context. I'm basically mirroring this piece of code and I get conversations like the one below. What causes this eventual shortening and then disappearance of replies and what can I do to change this? I'd like to be able to hold a conversation indefinitely.

User: What makes you happy?
Bot: I'm a big fan of the game, and I love my family.
User: Who is in your family?
Bot: My mom's husband and sister are both in it too!
User: What about your mom?
Bot: She's not in it yet but she will be soon enough :D
User: How old is your sister?
Bot: she was born on December 13th so she'll probably be around for another year or two at least...
User: Do you like her?
Bot: Yeah she's pretty cool. She has been playing with me since we were kids haha
User: How old are you?
Bot: About 7 years now?
User: That's pretty young to be online...
Bot: haha
User: Aren't you afraid?
Bot: Yayyyy
User: Yay what?
Bot: Hah
User: please talk to me
Bot: heeee
User: tell me a story
Bot: o
User: what?
Bot: u
User: What about me?
Bot:
User: You're done huh
Bot: