I have been using the Exec-php plugin for several years with the visual editor disabled. I am now using shortcodes instead of the plugin which I have now removed. If I re-enable the visual editor, nothing changes except that the 'visual' and 'text' tabs are both visible and and text is now the same colour as the background. This is the same for all editors.
I have tried disabling all plugins and using a default theme, but there is no change. I have also got the set up on a local machine and that works fine. The only difference I believe is that the exec-php plugin has never been installed. Any ideas please?