This incident is resolved now.
Unpublished tweet can be publish anytime: all you have to do is to update publishing hour.
Sorry again for the inconvenience.
Oct 10, 17:20 CEST
The issue has been identified and a fix is being implemented.
Oct 10, 17:13 CEST
We're currently facing a publishing bug for Twitter accounts: scheduled tweets are not published. (no problem for immediate or queued tweets.)
If you scheduled a tweet for today from 12:30 PM UTC, you might notice the tweet is still not published (still in blue within your calendar view)
If you need this tweet to be publish, we recommend to edit this item and postpone the publishing hour.
We're on it. It should be back soon.
Sorry for the inconvenience.
Oct 10, 16:49 CEST