Online PDF submissions failing this week.

Hi, I'm part of an agency that regularly has created and used acrobat forms to capture and submit data. We've been doing this for years using Acrobat Pro 11. This week we've suddenly had our forms (most of our forms haven't been modified in years) failing to submit. They get stuck with the load bar at half. Some users will eventually get an error "An error has occurred during the submit process. Unknown error.". The sudden occurance of this issue on old unchanged forms makes it seem like the culprit was some recent update with Acrobat or Reader.

Oddly enough, the forms WILL still submit fine still if you use Adobe Acrobat to submit them, but not if you use Adobe Acrobat Reader or Adobe Reader.

Further still, if you press the submit button a second time, while the form is 'hanging' at the halfway point of the first attempt to submit, it will successfully submit the form.

Any thoughts? What additional information can I provide to help troubleshoot this?