Symptoms
This issue occurs when the REG_MULTI_SZ value in the following registry path is malformed (for example, it's missing the second null terminator):
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Lync\<SIP address>\"Dismissed DelegatorList"\REG_MULTI_SZ
For more information about the registry value REG_MULTI_SZ, see Registry Value Types.
Resolution
To fix this issue, install the October 1, 2019, update (KB4484102) for Microsoft Skype for Business 2016.