@crigox: Thanks for your report.
This bug is tracked here:
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 the address you have used to register on this forum.

Trying to add Amazon S3 Server to sessions

WinSCP 6.0

Error message:
Operation not allowed on sorted list

Stapelverfolgung:
(001FA940) System::Classes::TStringList::InsertObject
(001FA940) System::Classes::TStringList::InsertObject
(001FA91E) System::Classes::TStringList::Insert
(00E6F5D2) TLoginDialog::UpdateControls
(00E6EEAA) TLoginDialog::LoadSession
(00E6E708) TLoginDialog::LoadContents
(00E70056) TLoginDialog::EditSessionActionExecute
(003F4F51) Vcl::Controls::TWinControl::WndProc
(0035D160) Vcl::Stdctrls::TButtonControl::WndProc
(003F50B7) Vcl::Controls::_17076
(003F4F51) Vcl::Controls::TWinControl::WndProc
(003F4598) Vcl::Controls::TWinControl::MainWndProc
(00213A0C) System::Classes::_18201
(00031331) USER32.dll
(000220E1) USER32.dll
(00021B65) USER32.dll
(0002BAAD) USER32.dll
(00077B5B) ntdll.dll.KiUserCallbackDispatcher
(00037A33) USER32.dll
(00020BC1) USER32.dll.SendMessageW
(0007345A) COMCTL32.DLL
(00083BC2) COMCTL32.DLL
(00031331) USER32.dll
(000220E1) USER32.dll
(00021A52) USER32.dll.CallWindowProcW
(003F5062) Vcl::Controls::TWinControl::DefaultHandler
(003F4F51) Vcl::Controls::TWinControl::WndProc
(0035D160) Vcl::Stdctrls::TButtonControl::WndProc
(00213A0C) System::Classes::_18201
(00031331) USER32.dll
(000220E1) USER32.dll
(000209A0) USER32.dll
(000204EB) 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.