This can be fixed by adding following to product.json:
"extensionsGallery": {
"serviceUrl": "https://marketplace.visualstudio.com/_apis/public/gallery",
"cacheUrl": "https://vscode.blob.core.windows.net/gallery/index",
"itemUrl": "https://marketplace.visualstudio.com/items"
}
This can also be fixed by copying product.json from an official build (in the .zip archvie, product.json is under /usr/share/code/resources/app/product.json which contains above lines). Depending on your distribution, exact location may vary.
More information: https://github.com/Microsoft/vscode/issues/1557