Patrick,
Is it okay if I send you my form? I am encountering this same problem. It looks like this thread is a few years old, but I was hoping to get my issue resolved.
Here's my situation...
I have a rather large form that I've been working on for weeks. I save all of my copies with a date stamp, so I do have an older version, but I lost everything that I was working on last week. I am in the process of making some enhancements and at the same time I have been doing some cleanup by labeling all of my fields and getting rid of old fields that are no longer in use. I think this messed something up because I am now getting the "Infopath has encountered an error. Operation failed", when I try to open the design view.
Please let me know! Also, I am with Great-West in Denver, so I know you will be visiting us soon for some training, but I have a deadline for this form of 9/1.
Thank you,
Tracey