I have the same issue on one server I upgraded this morning.
In the "server-manager", the main functions work as usually but, when I try to change a few settings, for example, add a new user, then, a blank "white" page appears.
I also installed on the same server the "sme-crontab_manager", that worked very well before upgrading the system this morning, and I'm not able to display any "crons" since I upgraded the server.
It's the same whith modifying "remote access" etc...