Cause of this error is someone try to delete E-Mail account while that E-mail account is using.
So files accountinfo and accountsetting on mail server are remove but other files can't delete.
And then if customer access to admin page the error "failed to read the account settings" will be show on the website.
When customer click E-Mail account and try to delete E-mail account, customer can't delete.
Solution to solve this problem is remove E-Mail account that has the problem on mail server.
But I can not access mail server by my internet. I will fix this problem on Friday morning.
Sorry for late reply.