This one is my favorite: DIGI Clock Widget. Are you saying that if you install it, then long-press an empty part of the homescreen and tap Widgets, you don't see this show up in the Widgets list?
That's odd, especially if there are still some system widgets you can add. Normally I would suggest a cache partition wipe, but I think all Nokia phones have seamless updates, which means no system cache partition to wipe. You might have to tray a factory reset. If it persists after a reset, then send feedback to Nokia about this.
Could be that they know there's an issue, so they are working on it before it becomes a critical problem with a million "My phone crashed!!" posts They may do another update & hey presto your widgets