Hi @TMoellegaard and @VPSJohn
The last MI version is the 1.8.3.1
If this version doesn't fix your issues, means the bug we fixed yesterday isn't the one you're experiencing.
We'd need more info about the errors you're getting.
Please paste if anything appears in the error log while completing the order fails.
If you don't know how to activate the error log, please read this.
If you don't feel comfortable sharing your site's data in a public forum, please open a ticket here.
Best Regards,
José Andrés