When creating a new user account or modifying the username of an existing account set with one of the following login types: Solabs Identity, OneLogin, or Azure, it is possible to get a system error if the SharePoint Access field is set to any other value than N/A.
The workaround for this situation is to first make sure that the SharePoint Access is set to N/A.
If it is not, modify the account to set the SharePoint Access to N/A without any other changes (make sure to keep note of the original value to revert to it).
Once this has been confirmed, proceed with modifying the username.
Once done, if the SharePoint Access had to be changed to N/A, modify the account again to set it back to its original value.
Important: This is also applicable when creating a new account with the SOLABS Identity Authentication Type. If the new account requires SharePoint Access, it should 1st be created with the SharePoint Access set to N/A, and once done, the account can be modified to set the desired SharePoint Access.