Errors such as "Error loading source type" and "Connection error" usually appears when adding ads-related data sources like the Adsense or Google AdWords API data sources. This is due to an ad blocker extension running in your browser.
Turn off or disable your ad blocker when adding or working with the aforementioned ads-related data sources. Make sure you refresh the page afterward.
Shown below are the screenshots of the errors that appear if you have an ad blocker running when working with the AdWords API and Adsense data sources.
And this is what they look like if you disable your ad blocker: