I am a happy fvwm user on an a somewhat old openSUSE 11.3 (I tend to upgrade only at hardware change). We recently have got some high school students for a stage, and put them on a freshly installed opensuse leap 42 workstation.
Since I wanted those working under my supervision to share my fvwm configuration (cuius regio eius religio 
), I installed (from yast2) the fvwm package. Most of the times it runs ok.
In a few cases I found that after logging out, the suse display manager seems frozen (the login mask does not respond). Until today I cured it with a reboot (and attributed it to the OS).
Today however I had the same problem and a reboot did not cure it. Even when the display manager is unlocked, one enters user and password and presses login and then it freezes. If I login remotely, I see that the fvwm session has started (a ps shows the X server, an sddm process, the fvwm and the dependent processes … but nobody appears on the display) … also a “who” does not show the session.
I am not familiar with this sddm (on my older 11.3 the way the X server starts the window manager is different). The Xsession error log does not show anything significant.
I was able to cure it after a reboot logging in under KDE then logging out. After that the fvwm session started normally.
Any idea ?