I'm sorry to hear you are experiencing this issue. I reviewed your account and I see you reached out to our Call Support team today and they have our upper level support taking a closer look at the issue you are experiencing. But they did mention they were able to find a workaround so you could send your email out.
Just in case someone else might come across this post, I wanted to provide some troubleshooting steps:
1) Try another browser. If it works in one browser, but not another, that at least narrows down that the issue is central to the affected browser.
2) Try Incognito/Safe Mode on your browser. It's possible your browser has some third party plugins or extensions that are inteferring with the site experience. Disabling these may resolve the issue.
3) Do an Internet search for a speed test. Constant Contact works best at 2 mbps upload and download. If it is running lower than that it could be contributing to the issue.
If these do not work there might be other methods we can try to figure out what is going on. Please email us at social_support(at)constantcontact(dot)com with your username, the name of the campaign you are having an issue with, and a reference to this post.