Hi Kees,
From the log file, it looks like restgui service is not configured correctly on the target server. Please check if you have set the correct handler class and turned on compression. Once done, please test if the following works fine:
http(s)://<TARGET SERVER>:<PORT>/sap/restgui/newsession
http(s)://<TARGET SERVER>:<PORT>/sap/restgui/state/text
Thanks
Chinthan