This error has been a real pain in the ass since before 4.7
If you have 4.9.0 and later, try logging in as host and clearing your cache.
Or get to your file system and delete the contents of all the cache directories in the Portals directory.
Finally, make sure that Ajax is enabled in the Host Settings.
If this doesn't work, don't contact me about your "ScriptManager must appear before any controls that need it" errors. I've endured enough pain.
Alternate Solutions to Consider