If there is an error 500, you could enable WordPress debugging and see the error in your debug log.
1. Once you spot the error, please send it to me so I can have a look.
2. The features you disabled use
wp_mail(). If mail is not configured on your server, this could be the source of the problem.
If you determine that this is the case, you could try setting up this plugin to forward your emails through a Gmail SMTP.
Please do let me know what error is written to your logs.