301-custom-images-at-scale - Internal Server Error

[301-custom-images-at-scale](https://github.com/Azure/azure-quickstart-templates/tree/master/301-custom-images-at-scale)

### Issue Details
Trying to deploy either single VM or VMSS from Linux image - getting error "Internal Server Error" at validation step

### Repro steps (*if necessary, delete otherwise*)
1. Start Deploy to Azure
2. Create a new RG in the same region were SA containing image is located
3. Put required parameters (all check marks are green). Linux image is generalized and copied as per MS manual
4. Accept terms and Click Purchaise
5.

Start with the linked 301-custom-images-at-scale template and reproduce the validation failure using the listed steps for a generalized Linux image, first with a single VM and then with a VMSS. Check the deployment validation response and parameters around the image and storage account. Done means the template no longer returns Internal Server Error, or the issue is narrowed to an Azure service failure with clear evidence.

Written by the indexing model from the issue text.