@jonathan.berry: Thanks for your report.
Most likely this issue:
Issue 2166 – Failure when selecting S3 protocol in Login dialog with non-existing .aws/credentials file
I'm sending you an email with a development version of WinSCP to your email address.

<Try to describe precise steps that lead to the problem (where do you click, what keys do you press, what do you see, etc.)>
<If relevant, consider attaching a session log file or a screenshot)>

WinSCP 6.0

Error message:
Operation not allowed on sorted list

Stack trace:
(001FA940) System::Classes::TStringList::InsertObject
(001FA940) System::Classes::TStringList::InsertObject
(001FA91E) System::Classes::TStringList::Insert
(00E6F5D2) TLoginDialog::UpdateControls
(00E6FB94) TLoginDialog::DataChange
(003F4F51) Vcl::Controls::TWinControl::WndProc
(003F0048) Vcl::Controls::TControl::Perform
(003F50B7) Vcl::Controls::_17076
(003F4F51) Vcl::Controls::TWinControl::WndProc
(003F4598) Vcl::Controls::TWinControl::MainWndProc
(00213A0C) System::Classes::_18201
(00040239) USER32.dll
(000371F5) USER32.dll
(00036DF5) USER32.dll
(0003A9BD) USER32.dll
(00073FBB) ntdll.dll.KiUserCallbackDispatcher
(00034C1A) USER32.dll.SendMessageW
(00072335) COMCTL32.DLL
(0007286B) COMCTL32.DLL
(00040239) USER32.dll
(000371F5) USER32.dll
(000368D2) USER32.dll
(00034A76) USER32.dll.CallWindowProcW
(003F5062) Vcl::Controls::TWinControl::DefaultHandler
(003F4F51) Vcl::Controls::TWinControl::WndProc
(003F0048) Vcl::Controls::TControl::Perform
(003F0060) Vcl::Controls::TControl::Perform
(003EDE91) Vcl::Controls::TControl::SetTextBuf
(00E73D21) TLoginDialog::TransferProtocolComboChange
(0035B5CC) Vcl::Stdctrls::TCustomCombo::Change
(0035B797) Vcl::Stdctrls::TCustomCombo::Select
(003F040D) Vcl::Controls::TControl::WndProc
(003F4F51) Vcl::Controls::TWinControl::WndProc
(0035B427) Vcl::Stdctrls::TCustomCombo::WndProc
(003F0048) Vcl::Controls::TControl::Perform
(003F50B7) Vcl::Controls::_17076
(003F4F51) Vcl::Controls::TWinControl::WndProc
(003F4598) Vcl::Controls::TWinControl::MainWndProc
(00213A0C) System::Classes::_18201
(00040239) USER32.dll
(000371F5) USER32.dll
(00036DF5) USER32.dll
(0003A9BD) USER32.dll
(00073FBB) ntdll.dll.KiUserCallbackDispatcher
(00034C1A) USER32.dll.SendMessageW
(000472B5) COMCTL32.DLL
(00041D59) COMCTL32.DLL
(00040239) USER32.dll
(000371F5) USER32.dll
(000368D2) USER32.dll
(00034A76) USER32.dll.CallWindowProcW
(003F5062) Vcl::Controls::TWinControl::DefaultHandler
(003F4F51) Vcl::Controls::TWinControl::WndProc
(0035B427) Vcl::Stdctrls::TCustomCombo::WndProc
(003F4598) Vcl::Controls::TWinControl::MainWndProc
(00213A0C) System::Classes::_18201
(00040239) USER32.dll
(000371F5) USER32.dll
(00036DF5) USER32.dll
(0003A9BD) USER32.dll
(00073FBB) ntdll.dll.KiUserCallbackDispatcher
(00034C1A) USER32.dll.SendMessageW
(000F1AAA) COMCTL32.DLL
(000ADD19) COMCTL32.DLL
(00040239) USER32.dll
(000371F5) USER32.dll
(00034F65) USER32.dll
(00034D2B) USER32.dll.DispatchMessageW

I know how to reproduce the problem or the problem happens frequently enough. I wish to be contacted by the WinSCP team to help resolving the problem.