if i remember correctly - during summer 2021 an update was made to change basics to the terrain-handling. the world was converted automatically to the new terrain-modification-model.

but valheim gets several major updates and patches from EA-launch till now... try it - but possibly your old world cannot be converted/updated to the last state if the savegame is too old - or some elements missing whatever.

but - i´m sure your char-file will work as expected. in case your old world is gone, start a new world with your old char.

greetings