When opening an existing web experience created using ArcGIS Experience Builder, the following error message may be returned:
404: Resource does not exist or is inaccessible.
However, the published version of the web experience displays as expected.
This issue is caused by the config.json file in the item resources being either missing or corrupted. You can check this by using ArcGIS Assistant to view the item's resources and verify if the config.json file is present or not, as shown in the image below.
If the config.json file is missing, it can be replaced by the following steps. This will not recover any changes made to the web experience after the last published version:
You should now be able to open the web experience in edit mode.
ArcGIS Assistant is a third-party tool that can be used for troubleshooting, but is not supported by Esri.