Hi @sparky, thanks for reporting this behavior. There was an update pushed out recently that is designed to prevent multiple instances of the designer being loaded for the same site on different browser tabs or in different browser windows.
I am here to help, just to confirm, when you say the site is not opened in another browser, do you mean that there are no other browser tabs with the same site loaded in the designer?
Is it possible for you to share the read-only link to the site? https://university.webflow.com/article/how-do-i-share-my-sites-read-only-link, if you wish to send me a private message with the site link, or send mail to firstname.lastname@example.org, we can look at that privately.
Did you by any chance get the error when pressing cmd+s or ctrl+s to save the site ?
Was there any other errors or strange things happen that you can recall, report?
Also, I would check is to make sure that your browser is running the latest updates, and to see if there are any browser extensions loaded that might be conflicting with the Webflow browser code.
A good way to check if browser extensions are conflicting, is to open the browser in Incognito mode (in Chrome) or Private Window mode (in Safari) and reload the site in the designer.
Thanks in advance,