FYI: I'm usin YouTrack Incloud.
I setup single "Fix version" field as a Sprint identifier across all projects.
Sprint naming is shared across projects but Sprint periods are not. For this reason, I have to open ALL agile boards and setup Sprint period manually.
Here's what I did.
1. In agile board 1, create new sprint.
2. Select 'Edit sprint' button (date shown) and click periods.
3. go to agile board 2
4. click newly created sprint, click 'Edit sprint' button, and click periods equally. It always has 15 days initially.
5. go to 3 for all other six agile boards.