The env var is 'INCOMING_CONNECTION_TIMEOUT'. refer : https://help.sap.com/viewer/4505d0bdaf4948449b7f7379d24d0f0d/2.0.05/en-US/0aac697f0cf7444193ed5eb0fc6e5bd0.html  

Solved by Fix of Portal Service.

Hi,michael.fallenbchelI have the same issue . I think this is caused by high resolution(for me , 4K) , and when i changed to 1080p , The SAP GUI works well.