Can't update users in CB backend

I have copied my development site to internet hosting service Siteground. Everything has been working for about 3 weeks. Today I noticed I cannot change any user details in the CB user backend on Siteground. Still works fine on development server. I have limited access to error files there so I am at a loss as to what's happening.
1. Change anything in CB user edit and throws me to Joomla Control Panel without changing it.
2. I can change things in Joomla user.
3. I did change "acceptedterms" recently to be required but this is identical in development. Removing requirement makes no difference.
4. I can change the user details from the front-end and they are updated.

I see this issue has been raised before but those solutions are not helping

Nevermind! I just found I had added a "Custom HTML" field to the public site (Siteground) only. It was not required - simply html that created a button on the profile for certain users. When I un-published the field, I could successfully update user details on the backend.
An un-wise request by site owner for a convenient link to the backend, but I am still curious why that would kill CB user updates!

Unclosed HTML tag or a <form> element in your Custom HTML field can break the profile edit form in frontend and/or backend. My guess is your HTML was invalid.