This is most likely due to no Earliest Sickness Date existing in SelectHR. This field requires a value to be applied for the integration to operate correctly.
You can modify the earliest sickness date by following the steps below.
In the Employee zone, click Lookups.
Click Pay, then Payrolls.
Click into the Payroll Name, which needs to be changed.
Change the date under Earliest Sickness Date.
If the advice above still hasn't fixed the issue, please raise a new case with support, including examples of affected employees and the dates of the respective sicknesses.