The invalid grant error comes from a bad Authorization token from your Quickbooks company account. These are normally seen when Quickbooks makes an update or does something that requires other software (SingleOps) to renew the token.
How to Fix
1. Locate your Account Settings page from the Admin gear icon on the left hand side of your SingleOps account. From there select Quickbooks Online.
2. You will most likely need to disconnect your QBO account before reconnecting.
3. Now you will need to reconnect you QB Online account. This will force the system to retrieve a new Auth token so you can begin syncing again. You will need you QB Online admin credentials to complete this step.
4. Once this complete you can view any remaining errors from you Reports tab --> Quickbooks sync report. Any errors with the "Invalid Grant" Message can be "resynced." After you resync the errors you should refresh the page to confirm they are resolved.
5. If you need any help or require further assistance with this please contact Support@singleops.com