Associated error messages:
WinSCP.SessionLocalException: WinSCP process terminated with exit code 3 and output "", without creating a log file.
System.TimeoutException: Timeout waiting for WinSCP to respond - Log file C:\...tmp was not created in time, please make sure WinSCP has write permissions to the folder
The bug was always present in WinSCP. But until the 5.0.8, with its "Dynamic base address" executable flag set (Windows 8 certification requirement), the bug happening was not plausible.
While the fixed versions no longer gradually exhaust system resources, they cannot recover resources exhausted by previous buggy versions. So it's still necessary to restart affected machine to recover the system.