I can't even rename them... Could it be a virus? When trying to open them i get "We can't play this file". They are in the Videos folder. I was told that I will be able to delete them if I connect to a PC via cable....
Welcome to Android Central forums. I'm not sure how many users you might bump into here, I am not one, but I found the following online. It sounds a bit different than how you describe. Let us know if it works for you.
To delete media from a Viber chat, you can do the following:
To permanently delete your data from Viber's servers, you can deactivate your account. Deactivating your account will also delete your profile information. However, Viber may keep records of some actions you took for up to 24 months after deactivation. [2] You can also watch this video to learn how to clear cache memory on Viber: