Although this is now a non issue for me, hopefully this helps. I temporarily had this exact same issue last night using 0.12.3. This was a day after I had installed then uninstalled Dataview, Templater, Outliner, and Sliding panes on this particular vault.
Also, not sure if it is related, but it happened right after I set the hotkey for toggle heading to Alt H on Windows 10. I am not sure what Toggle Heading is supposed to do, but Obsidian wasn’t responding to that hotkey either. After a little while Enter would work again then stop.
I had no plugins or custom css on the vault. I uninstalled then reinstalled Obsidian and the Enter problem went a way. I will do more testing when I get on computer later today and look into the Toggle Heading thing.
Thanks.