The article doesn't say because the filing which it's based on didn't specify the details of the dispute or reproduce the full letter that Nokia sent. We don't publicly know yet, although I'm sure that if this goes to court we'll have an opportunity soon to dive into the details.

Based on the type of service that Reddit provides, there's almost no reasonable patent that they would have infringed from Nokia. So it could be something related to video compression/delivery, or some very vague social media feature that's commonplace but somehow patented.

https://www.sec.gov/Archives/edgar/data/1713445/000162828024...