It is probably related to change Teamleader took into production last Saturday September 10 around 12:00. See Laden Teamleader via Data Hub geeft error itgenobr024.
When using the Data Hub with the headless Teamleader OAuth provider, the software will no longer connect.
A new version will be released once resolved.
Please note that a new version 20.2.xx will require Microsoft .NET 5 to run on Linux, available on Download .NET 5.0 (Linux, macOS, and Windows).