Other reports of the model page failing to load have been a result of an error being introduced into the file in the jsondb folder that stores the items information org.openhab.core.items.Item.json. Pretty much if the UI fails to parse the full information about any single item, the model construction fails altogether.
If you only have a few items you might be able to look through this file and find the error (but please see Editing the JSONDB a last resort in this post first). Or you can try to restore one of the older versions of that file that’s in backup in that same directory.