In order to test this further, I made a copy of my form to use for testing, and routed the user task to that form instead.  Then I made a small change to confirm that when I opened the form from my inbox, it pulled up the new test form instead of the original version.

Then I started systematically removing parts of the form and testing to see if the error went away.

I deleted all my CSS, my Javascript, the Lookup Rules, the Field Rules, and the error persisted.  Then I went through and removed all my fields (a couple at a time) and testing in between, and the error persisted.  I am now to the point that the form is blank, all that exists is the built-in comment field, and the approve button.  And I'm still getting the error!  The only thing left on my form is the variables, which I can't delete since that would break my real version of the form.

I've already confirmed that none of my variables are unnamed (I had that issue once before), and as I mentioned, the form does progress from User Task 1 on to User Task 2, just not from User Task 2 on to User Task 3.

I don't think it's my actual form causing the issue, since I deleted the entire form and still had the issue.  So I'm down to:
1. An issue with my process, but I don't see one.
2. An issue with my variables, but I don't see one, and it works the first time.
3. A system bug of some sort.

I don't know where to take this from this point...

If you can reproduce with a specific process, can you open a support case with your VAR and provide the downloaded process xml for us to do trouble shooting?

Providing a copy of my process to our VAR was not going to be a very easy option, it's pretty complex with Workflows, and connections to multiple databases.  So I tried replicating the error with a simpler process, and was able to do so.

Here's the steps I took:

I made a Process Called "Error Testing".  The process had one form, named Test, with no fields, no lookups, no CSS, and no Javascript.  The Process Disagram consisted of 5 steps:
1. Start Event
2. User Task - using the Test form, including the Submit button, and Assigning to my own user - all other values are at default settings.
3. User Task (2) - using the Test form, including the Submit button, and Assigning to my own user - all other values are at default settings.
4. User Task (3) - using the Test form, including the Submit button, and Assigning to my own user - all other values are at default settings.
5. End Event

Then I created a Workflow that consisted of one sole task, calling "Invoke Business Process" and loading the "Error Testing" Form Process.

I ran the Workflow:
1. The Form "User Task" was created and placed in my Inbox.
2. I opened that form and clicked Submit, which worked.
3. The Form "User Task (2)" was created and placed in my Inbox.
4. I opened that form and clicked Submit, and then I got the same error I was getting in my complex process.

I can provide our VAR with this process and Workflow if that is still the best course of action, but perhaps this information is sufficient for someone to replicate the issue?

Thanks @████████!