I am sorry to hear about the issue you experience.
The error you are getting when creating a new user might be related to a known issue that users with a valid GUID for their username can not be saved. The bug is logged in our feedback portal. Here is the link
where you can track its progress and vote to increase its priority.
If this is not the case, can you please give some more details about the custom fields you have created for the basic profile type and the exact steps you take to create the user. This way we will be able to test it following your steps and check if the issue will be reproduced on our side.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items