The documentation for running mojoPortal on Azure is for the old Azure portal. Azure has changed a lot since then and I haven't had time to fully evaluate it recently and provide updated documentation for it.
I do see they have a new web sites feature and you can install the latest 22.214.171.124 version of mojoPortal there directly from the Web App Gallery. I just tested this myself and it worked very smoothly to get the site up and running. But I'm not sure yet whether it really works correctly, I'm need to evaluate it longer to find out some things. For example if I upload an image from the mojoPortal editor it gets stored on disk on a virtual machine, but I'm thinking it is not statefull, the vm will lose any files added from the web if it gets rebooted or swapped out for another instance. But I could be wrong, I've uploaded an image to see if it does persist or not but it may be a while before the machine is recycled since it is a shared hosting not a dedicated vm. So at this point I don't yet have advice on the best way to run mojoPortal in the latest Azure or whether it really works as it should installed in Azure Web Sites.
One way might be to use a high availability drive with multiple web roles mounting the drive and running the site from the files on the shared drive. That seems to be what they do it in the DnnAzureAccelerator, which is open source and could possibly be used/modified to install mojoPortal in a similar way.
500 is just a generic error, all server errors generate a 500 http response status code. In order to offer any help one would need to know the underlying error detail.