Performing configuration sync blocks subsequent printing.
There is a full workaround for this issue:
After performing a configuration sync you will be returned to the "Staff Logon" screen.
Fully close the VestiPOS App by double tapping home button and swipe up on the VestiPOS app preview.
When you reopen the VestiPOS app printing will be working again.
We are treating this as a critical issue. Updates will be posted here:
2019-05-21 15:06 UTC: Our development team are currently working on a permanent fix which we intend to submit to Apple today. We will be expediting the update with Apple to get it through their approval process as quickly as possible. We'll update further once it's available but until that point please use the workaround above.