Hey @Harriette Blye3, so sorry for the trouble! Are you using any personalization codes (like for first name)? If so, could you please verify that there are no syntax errors? (Sometimes the easiest way to 'check' that is to just remove the code entirely, and then re-add it.)
If that doesn't help, could you please test from a private browsing/Incognito window? If it works there, that might indicate that there's something in your cache or cookies that is interfering with ConvertKit on the regular browser window. And if so, clearing your cache and/or cookies for your regular browser window should resolve it (just make sure you back up any changes you had made to your sequence since last being able to save, since they'll be lost).
If the issue still persists, could you please reach out to our team via email@example.com (or via the in-app messenger in your account) with a detailed description of the issue, and the names of any affected sequence(s)? Our team'll take a look to see if there's something else causing this!