Originally posted by Magsu:

I'll do some network checking with her later to see if her connection is really that bad that it could be affecting the whole server.

Image taken from MonsterHunter Rise guide about connection errors: https://steamuserimages-a.akamaihd.net/ugc/2019334537282658173/058CD34E5D17C3A3C60F27AF67AD181EC64485F6/
https://steamcommunity.com/sharedfiles/filedetails/?id=2915995696

In an extremely over simplified generalization; peer-to-peer games, like Valheim, are more likely to equalize to the lowest common denominator. AKA the worst connection, because of how the data packets get sent back and forth between every connection constantly. If they start dropping packets or failing to send, the whole thing breaks down.