Error 500 might appear when you're trying to change your email address.

An email address can be associated only with one Meister account. A 500 error occurs when you try to change your e-mail address to an account that already exists. To resolve this error, you will have to delete the Meister account registered to the email you're trying to use.

To solve this issue: 

1. Close the Meister account that is associated with the email address you want to use by following this link. Note: When you close your account, your account data will be deleted. 
2. Change your email address to the now available email address. Find out how to do this here.