ScheduleOnce has a known issue where when it cannot sync to a user's Salesforce user it displays an error message claiming a 'temporary connection issue'. The recommended resolution according to Oncehub support is to remove and reconnect the Salesforce sync for that user. Basically turning it off and on again.
1. The effected user should login to ScheduleOnce, and access their booking page:
You want your booking page specifically, and not the update your integrations quick link.
2. Once on your booking page (if you have multiple you should only need to chose one) go to integrations and remove the CRM (salesforce) Integration.
3. Login to Salesforce as usual.
4. Go back into ScheduleOnce and set up the integration anew from your booking page.