System.IO.IOException Possible Fix

Everyone I've seen having the System.IO error has gone through the same process of uninstalling everything or disabling onedrive, but something I found that worked that I haven't seen anywhere yet.

Head to "Documents\My Games\Terraria\Players" and find the BAK file causing the issue (for my friend it was a character he made one year ago so check older players first)

Try to delete the file, if you get the same error you get when trying to join the game then you've found it.

Take the BAK file and move it into a sub-folder named "Disabled" or "Broken" and delete the folder that goes with the bak (if you can't delete the file that goes with the bak, just rename it)

That should fix the issue for you if cloud saves are the problem.

broken folder

[Strayn]

this fixed it