I’ve never seen that “expert mode” option before. Is this something you’ve used before and it works? What’s it do?
I’m on the OH 4 snapshots and it doesn’t work for me. I get only a slightly different error.
There was an error generating the items definition: TypeError: Cannot read properties of of undefined (reading $oh)
Nothing appears in my openhab.log.
I can’t say if it’s a regression or if it’s an incomplete feature. But I recommend following How to file an Issue on the openhab-webui repo.