I checked the command line as you suggested, but it is not truncated in Process Explorer. I also can open an identically named file when it is in a less deeply nested directory (e.g. C:\Temp). It seems that the apparent limit of 100 was just a conincidence, because when I try to reproduce the problem from WinSCP now, it is 111, maybe because I used a different remote directory then. The actual limit is probably the whole path length, which hit 260 (=MAX_PATH, cf. https://docs.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation
). I have set the registry key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem\LongPathsEnabled to 1 to enable longer paths, which might also play a role.
So I suppose that the actual problem lies with both editors using an older API which doesn't support long file names, not with WinSCP.