I was having the same problem with contact form 7 and also host with Siteground. The solution for me was that the ‘Store uploads in this folder’ setting under Settings > Media was incorrect. I simply deleted what was in there and hit save. I migrated the website from another host and guess this setting didn’t get updated. If you can’t access the dashboard, then just manually disable contact form 7 temporarily. I hope this helps others, sorry if it doesn’t work for you.
