putty.exe cannot be found, .ppk cannot be opened

After some time WinSCP is running I'm prompted for password when saving file edited in external editor. It's due that my ppk file cannot be opened. Same I cannot use ctrl+p to start putty it said that it cannot find putty.exe

After program restart problems gone.

-using ppk for login on ssh server
-ppk and putty.exe are in same dir as WinSCP
-WinSCP portable
-I'm using same ppk for multiple sessions at same time
-I'm starting WinSCP from toolbar in TotalCommander(run in dir is same as program's dir)
-I use SCP protocol

WinSCP 5.5.0 (b3839)
This problem started after upgrade from 5.1.2, happened twice in ~2 weeks
Win 7 x64

Thanks for your report.
I have sent you an email with a debug version of WinSCP to address you have used to register on this forum.