I am using the default theme. Since widget/sidebars are dependent on theme that you are using, so it may be an issue with the theme. Not sure though. Can you please switch to the default theme and try rearranging the widgets to see whether it solves the problem or not?
There may be some other factors leading to this issue. Try below options:
- Open the Screen Option on Widget panel above and disable accessibility mode.
- Check for magic quotes. Try turning it off and restart the server.
- Try increasing the memory limit by adding the following to wp-config.php file
define('WP_MEMORY_LIMIT', '32M'); or higher value as per your requirement.
- Resetting the plugins folder by FTP or phpMyAdmin? Sometimes, an apparently inactive plugin can still cause problems.
- Re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress?
- Talk to your hosting provider.
- Please have proper back up before proceeding as above.