So, the issue then is with the Keywords list. It has a lookup that uses the Referrals Library.
Your form - what is submitting to the Keywords list? Valid ReferralIDs? That is, does your form give the user a dropdown to select the ReferralID from?
I think I'm doing a poor job of explaining what I mean, but I will try again.
Currently, you are probably submitting 1, 2, etc. to the Keywords list ReferralID field from your form field.
When the ReferralD column in the Keywords list is just a text field, your submit works without issue. SharePoint is expecting some text, and that is what you are providing.
Lookup columns require the data be in a different format. The format that the lookup column wants is the ID of the lookup item, followed by a semi-colon, a pound sign, and the display value.
If your Referral Library looked like this:
ID Name
1 First Item
2 Second Item
You'd want to submit:
1;#FirstItem
To successfully populate the lookup from your form.
This is why I'm suggesting you add another field to your form data source and use a formula to set it to the value SharePoint expects (whether a default value or a rule) and submit this new field to the ReferrallID column in your Keywords list.