Update (9th December):
- 2.13.7 has been released to address the bug with the event list widget and WordPress 4.4
- A bug in WordPress 4.4 has been reported (and a patch submitted) which may cause a warning on the single event page. A workaround is enable Tags in Settings > Event Organiser > General
- No conflict with JetPack was found
Event Organiser had been tested prior to WordPress’ 4.4 release yesterday. Unfortunately that testing did not uncover a defect with event widgets with titles when running on WordPress 4.4.
You can fix the bug yourself if you feel comfortable doing so by applying the patch (it is a one-line addition) or you can resolve the issue by removing the widget title.
Lastly, there is a potential issue with Event Organiser & JetPack on WordPress 4.4 – this report is as of yet unverified, but if you have encountered any issues with Event Organiser & JetPack, I’d encourage you to get in touch.
Apologies for any inconvenience caused.