Sounds like you hit the max allowed save-file size for your browser (or you ran out of disc space). What browser are you using? And how big is your savefile when you download it?
Viewing post in Rogschard, Lords and Loyalty comments
Well, that sure is annoying. Tried to replicate the bug by downloading OperaGX myself. Seems to work without a hitch on my largest savefiles. Also could not find any specific developer question related to OperaGX file storage. For now I am out of things to try, if the problem persists, all I can recommend is using a different browser. Thanks for reposting the issue! Sorry I can't find a fix for you right now, maybe something will come up in the future that explains the problems