Steven Zahm@ Kate
If you give me a temp admin access, I’ll take a look. The login details can be posted here as a private reply.
The issue happens as you are typing, correct? I do not see how that would be possible unless another plugin or theme were causing the issue. What about the theme? You said not plugins were added. What about plugin or theme updates?
Can you also try with a browser with javascript disabled? A lot of stuff will not work on the add/edit entry admin page without javascript but if it does not happen with javascript enabled, then you know it has to be another plugin or the theme.
Another thought… what about browser plugins? Please try replicated using a private browsing window. Most browsers, the private browsing window does not load with browser plugins enabled.