Without further details it’s hard to guess. The first thing to check is that the openhab user has read permissions on the .items files (any file in /etc/openhab2 really). The second thing to check is openhab.log for any errors. There could be a syntax error in your .items file that is preventing it from loading.
The loading of .items files is one of those things that hasn’t really been touched in years so I doubt you’ve uncovered a bug or instability there, but that’s always a possibility.
Because all you have to to is save the file. That’s it. It will get picked up by OH, and if there are no errors in the file, loaded and your Items will be created in memory.
There isn’t anything you need to do. If you save a .items file in /etc/openhab2/items, the openhab user has permission to read that file, and the file has no major syntax errors, that is sufficient.
I’m sorry you find it really hard to believe. But unlike many open source projects, openHAB is 100% volunteer driven. And it’s an old project with a lot of history.
The volunteer(s) who created PaperUI and contributed it to openHAB (Eclipse Smarthome really) built PaperUI using a framework that none of the other maintainers know or have a desire to learn how to work in. And that developer abandoned the project. So now we are stuck with a UI that we can barely keep running let alone add features to.
But there is work ongoing to replace PaperUI (and several of the other UIs as well) for openHAB 3. In the mean time, PaperUI is all we got, as incomplete and broken as it may be.
Start small. “Just want to control Fritz switches with Harmony though Hue Emulation” is almost an end to end project utilizing everything that OH has to offer short of persistence. This isn’t really something you can pick up and get working in a couple of hours. So rather than approach it as an end-to-end all at once, break it into smaller pieces and get them working first. Build up gradually like Vincent suggests.
Pretty much everyone on this thread is actively using .items files and have been doing so for years (largely because of the PaperUI issues you site). And we haven’t experienced nor seen reported of any problems like this.
What’s in your logs? What is the permissions on the file?
It’s helpful to post code like this using How to use code fences, especially for dealing with syntax errors like these. The forum likes to make everything pretty which can mangle the code. In this case, your quotes should be "Switchable" but they are showing as “Switchable” (see the difference?). The latter version cannot be parsed.
If there is a syntax error, you will see it listed in openhab.log with the line and column number of the source of the error.