Good morning Ken. It sounds to me like either the flexibility theme is the issue. Or one of your other plugins that you are using with the flexibility theme is the issue.
Especially if you have a page that works with the theme, but another that doesnt. Then its definitely a plugin.
So what we want to do is deactivate all of your plugins and then see if you can add the link normally.
If you can, then we should activate half of your plugins and check if you can add the link normally.
And keep turning on half of the plugins ( and keeping track of what you turned on ) Till it stops working. Once it stops working.
You know the issue was with one of the last few plugins you turned back on.
Then its a matter of whittling them down one at a time. Heres the FAQ page on plugins, as a reminder of how to get to your WP plugin panel, in order to troubleshoot.
If you have further questions or run into any issues, let me know!