OK, thanks for the information.

The editor doesn't matter. Tried it with Windows Notepad and Notepad++. It seems that Windows is not capable of using long paths in arguments (or in the Windows API when the program tries to open that file).

I have set in my registry to 1. But this only leads to the effect, that double clicking the file will use as system call instead of the full path.

What external editor are you using? How exactly do you "open the file locally" to circumvent the problem?

As a workaround, you can turn off Append remote path to temporary path:
https://winscp.net/eng/docs/ui_pref_storage

Most of Windows still has problems with long file names. Even though WinSCP is able to download and show them in internal editor, it's not possible to use an external editor right now if the path name gets too long.

If I open the file locally, Windows is circumventing this by using

C:\Users\myusername\AppData\Local\Temp\scp32179\plugins\servlet\CONFLU~1\default\Global\LIV3DEV\LIV3DE~1\Products\WAREHO~1\WAREHO~1\TECHNI~1\PACKAG~1.IUM

instead of

C:\Users\myusername\AppData\Local\Temp\scp32179\plugins\servlet\confluence\default\Global\LIV3DEV\LIv3 Development\Products\warehouseNAVIGATION\warehouseNAVIGATION MSP\Technical Specification%3A warehouseNAVIGATION MSP\Package.Sub.TruckCommunication.GetLoadSensorOrder.iuml

WinSCP is trying to open the editor using the full filename instead of the 8.3 formatted version which fails.

I'm using WebDAV with Confluence in this case and have therefore no influence on the length of the filenames and resulting paths.