Retrieving objects from `DbContext` using ToArray/ToList throws an `InvalidOperationException`

Question

I have a c# DbContext defined like this:

public class DataContext : DbContext
{
        public DbSet<Account> Accounts { get; set; }        
        ...     
}

In another place I have instance of DataContext which I use to retrieve all my Account objects - and I expect many of them, but the code below:

var accountAll = context.Accounts.ToArray();

throws an exception:

System.TypeInitializationException: 'The type initializer for 'Microsoft.EntityFrameworkCore.Query.QueryableMethods' threw an exception.'

Inner Exception
InvalidOperationException: Sequence contains more than one matching element

Can someone understand why exception is thrown?

Class Account:

using System;
using System.Collections.Generic;

namespace WebApi.Entities
{
    public class Account
    { 
        public int Id { get; set; }
        public string Title { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Email { get; set; }
        public string PasswordHash { get; set; }
        public bool AcceptTerms { get; set; }
        public Role Role { get; set; }
        public string VerificationToken { get; set; }
        public DateTime? Verified { get; set; }
        public bool IsVerified => Verified.HasValue || PasswordReset.HasValue;
        public string ResetToken { get; set; }
        public DateTime? ResetTokenExpires { get; set; }
        public DateTime? PasswordReset { get; set; }
        public DateTime DOB { get; set; }
        public DateTime Created { get; set; }
        public DateTime? Updated { get; set; }

        // Notification section
        public bool NotifyWeekBefore { get; set; } = true;
        public bool NotifyThreeDaysBefore { get; set; } = true;
        // End of Notification section

        public List<Schedule> Schedules { get; set; }
        public List<Function> UserFunctions { get; set; }
        public List<RefreshToken> RefreshTokens { get; set; }

        public bool OwnsToken(string token) 
        {
            return this.RefreshTokens?.Find(x => x.Token == token) != null;
        }
    }
}