Case sharing turned off but still seeing error

I turned on Case Sharing on our app in the hope that we could have better work flow for our surveys. However, in turning it on and creating the user Group, it has made the app unusable for our field team as the following error message comes up:
"Error: The case sharing settings for your user are incorrect. This user must be in exactly one case sharing group. Please contact your supervisor"
I turned off case sharing and deleted the user Group.
The field user uninstalled and re-installed the app, but he is still getting this message.

Case sharing is desirable but not essential to our study which has not started yet, but will start very soon. I would rather turn it off and not have the bother!

Any help in solving this problem would be appreciated!!
Thank you