@kalusha – is this the plugin you’re using? WP-Members
I’ve installed that plugin and played around with it to try to reproduce the error. Everything appears to be working fine with both the default /wp-login.php?action=register form and the in-page register form for restricted pages.
If you can provide the exact error message to me or @robin-w we may be able to make Style Pack work with the WP-Members plugin.
Enabling Debugging may help you get the error. You can temporarily enable debugging by adding the following lines to your wp-config.php file:
define( 'WP_DEBUG', true );
// Enable Debug logging to the /wp-content/debug.log file
define( 'WP_DEBUG_LOG', true );
// Disable display of errors and warnings
define( 'WP_DEBUG_DISPLAY', true );
@ini_set( 'display_errors', E_ALL );
define( 'SCRIPT_DEBUG', true );
And for better control over debug messages, I recommend this plugin – DebugPress