Publishing membership UI error

Hi, a friend of mine was using Debian Stretch 9.13 and latest .deb from Cesium (she tried 1.6.12 and 1.6.8 with same problem). When clicked in the "Register as member", she was shown the 3 steps wizard, but when pushing "Next" in the second step... the button does not react... till the 9th or 10th time of clicked and then just a part of a page appears or is shown(like if it gets stuck), see screenshot... and it remains this way without allowing to go next or publish the membership.

She launched cesium --debug to see wether there was some error in the back when encountouring but it showed nothing so I guess it is just some UI error.