I would just like you to know that bx errors are usually temporary and are automatically resolved. If you're seeing this error post 72 hours you may want to try the following steps in addition to the ones mentioned above-
If you're using a third party theme, please try reverting to the default Blogger theme.
Try to revert widget to default. Before reverting the widget to default, backup your blog theme and then go to, Blogger Dashboard >> click on Theme >> Edit HTML >> Revert widget to default.
If the above steps don't help, try switching to draft mode and then change the theme and see if it helps. Here's how you can switch to draft mode:
Login to Blogger.
Click on Settings.
Click on User Settings
Use Blogger Draft > Yes.
Click on Save settings.
Hope this helps.