For starting my trip in OH3 I made some Things, Chanels and Items on tha same way like in I did in OH2. Than I tried to build the modell. When I created Equipments for the same items I created in the begining the semantic structure died and does not load anymore when and I clik on ‘Model’ menu point or try to Pick an Item from the model in a rule.
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 resortin 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.