Some days ago, I tried to install a widget in one of my custom templates and then I got this error (bX-xxxxxx). I tried to solve the issue by myself by doing some random things and I was finally able to solve the problem. Some people also face this problem when trying to upload a custom Blogger template. Here are the techniques you can try.
1- Change ids of Blogger widgets if you can't upload a custom xml template.
This one is a classical method and trusted by many Blogger experts, to do this follow the steps:
1- Open the downloaded template in Wordpad or Notepad++ (Download Notepad++).
2- Search (Ctrl+f) for widget id=
3- You'll find something like <b:widget id='example1' just replace the number '1' with some other number for example 5, 9, 15 etc. So you can make it <b:widget id='example5', <b:widget id='example9', <b:widget id='example15' etc.
4- Find all widget id= and repeat the step 3 but don't repeat any number i.e. If you've assigned a widget number '9' then don't use '9' again.
5- Once you've changed the ids, save your template and upload it by going to Blogger -> Layout -> Edit HTML and upload.
2- Clear your browser cache & cookies if you can't add/remove/edit widgets.
Google has provided detailed instructions for all browsers to clear cache and cookies, you can see it here:
3- Use some other browser if you can't add/remove/edit widgets.
This is also a good method. You can also try to work in some other browser. For example if you're working in Internet Explorer and continuously facing this problem, switch to Mozilla Firefox.