Problem: You are using WordPress’ great feature of automatic theme or plugin updates, but the AJAX throbber doesn’t stop moving – your site seems to be stuck. You hit the reload button of your browser and this is what you get:
“Briefly unavailable for scheduled maintenance. Check back in a minute.”
Solution: Open your (S)FTP program and delete the file called “.maintenance” in the root of the WordPress folder. This lets WordPress ignore the failed update, which you can then try again. It might be useful to update plugins one by one, or to check if some of them have become unstable.