I found out that the plugin sets the wrong folder as plugin-folder in some cases, when installing it as mu-plugin.
I solved it by changing line 61 in simple-fields.php. I have tried to install it both as normal plugin and as a mu-plugin, and it seems to work.. *touch on wood*
Here is the new line 61
define( "SIMPLE_FIELDS_URL", plugin_dir_url( __FILE__ ));
Maybe a thing to change in the next release?
Tack o hej!