Error qualifying a lead: Value cannot be null. Parameter name: g

Good day to all,

We've been having an issue when attempting to qualify a lead, it's consistently failing on every lead I've tried.

System.ArgumentNullException: Value cannot be null.
Parameter name: g

I've taken the following steps to try and figure out what's going on:

• Disabled all Plugins and workflows related to QualifyLead, Create (Opportunity) and Update (Opportunity)
• Verified that my Customizations.xml has no placeholders (as per https://community.dynamics.com/crm/f/117/t/177705)
• Attempted to check for relationships that are null that shouldn't be, I may be missing something here though

I'm on CRM Online 2016 Update 1, so my ability to look into errors is somewhat limited.

I've provided the request xml and the response from CRM here:

https://gist.github.com/AtomicBlom/fc96e868fb8e42d5a06d52340769ffe9

I'd appreciate any information you may have into this issue. I'm also mighty curious to know what the actual CRM qualification process is, what fields does it use to create the opportunity/contact?

Thanks,

Steven