ThaNerd

Actually, for some reason, it started working after I set up image magick on my server. GD was already active, but actually, your plugin called wp_get_image_editor, and that method returned wp_error telling “image_not_” something…

In case other people have the same problem as me, the solution was to install imagick.
Default PHP version on OS X Yosemite is 5.5.14, so using homebrew, you just add the tap “Homebrew/php” and install php55-imagick, then you add the imagick extension in your /etc/php.ini

I posted a longer information on my blog, in french

I have the exact same problem… Except that having much lesser visits than you, the “reset to no theme” happens lesser often.

I carefully checked my error logs and see absolutely nothing explaining the cause of this.