With the update of the Journals plugin (to ver. 2.0.+), the Daily Note button on the Note Toolbar is broken.
Solution
"commandId": "journals:journal:calendar:open-day"
must be replaced with…
"commandId": "journals:personal-daily:open-today's-note"
For that, close Obsidian, head to .obsidian/plugins/note-toolbar, open data.json with any editor of your choice, and exchange the line above with the one given for all occurrences (with CTRL+H
).