Form Error: Invalid text value

Ok, I'm stumped on this and I'll try to give as much information as possible. We are in Office 365 and have been able to recreate this issue in Chrome as well as IE.

We have a form that has the following fields:

• Title - single line of text. Default value is ARC Meeting: [Todays Date]
• Meeting Date - default value is today's date/time
• Scribe - people picker, default value is current user
• Facilitator - people picker
• Attendees - Multi-select choice (checkboxes)
• Additional Attendees - People picker that allows multiple selections
• Agenda - Multi-line rich text
• Discussion Items - Lookup to other list on this site that allows multiple selections
• Meeting Minutes - Multi-line rich text
• Action Items - repeating section
  • Item - single line of text
  • Responsible User(s) - people picker that allows multiple selections
  • Goal Date - date field (no time)
• Attachments

There are Save as Draft, Cancel, Save and Submit buttons.

The Problem:

When some users submit sometimes (not consistent) they receive the following error: 

I have no validation occurring anywhere; no JavaScript, no rules, nothing. Also, no field is highlighted when this error appears. 4 people all with the same permissions (in the same AD administered group) all get a different result, even when using the same browser. I did have some Javascript in the form, one piece checking the checkboxes by default on a new form and one piece forcing attachments to open in Office Online. I removed all JS and still have this problem, again, intermittently.

Progression of events:

• I tested the form, had no problems, rolled it out
• First user tries to make a submission, receives this error. When this error happens, all his formatting is removed from the rich text fields. Check user permissions, seem fine. Have another user in the same group submit with no issues. Come back to this user, try to have him submit a blank form (there are no required fields), receives same error. User is eventually inexplicably able to submit. I am still unable to recreate the problem with my credentials.
• Another user says they have the same issue. I mention the Javascript, he disables UBlock add-in, and is able to submit.
• Third user has the problem: I remove all Javascript from the form. User can now save as draft. Later mentions she still cannot "Save and Submit."
• Today, revisiting the issue with the third user, she submits the form several times, each time adding something (ex, only title the first time, second time title + date, third time title + date + scribe, etc). Is unable to submit after adding "Discussion Items." We realize that one discussion item has an ampersand in it (title of a request in another list). Once removing that discussion item, user is able to submit.

These chain of events don't seem to make total sense. User 3's problem can't be the same as user 1's since user 1 couldn't even submit a blank form. Thoughts? Things don't seem to 100% add up unless I'm missing something.