Cannot share video received in Signal

I receive a video in the app Signal, but I am unable to share this video. I am able to save it in the app Photos, but I am still unable to share this saved video with any app (including Signal, WhatsApp, ...). However, I am able to share photos received in Signal. The error message reads something like: "cannot prepare attachment" (translated) I ask this question for a friend, so I don't use an iPhone myself and am not familiar with it. There aren't any other problems (I could share videos received in Telegram). Does it have to do with app permissions? I am grateful for any ideas.