File upload in RFC function: We call a function via RFC that uploads a file. This works well since we use the "-dialog" option in the GuiXT call statement. But when we perform the RFC logon with a different user, that is a "system" user, not a "dialog" user, the upload no longer works. How can we solve this?

"System" type users are not permitted to perform dialog functions. If you need to logon with a "system" type RFC user  I suggest you avoid the upload completely. Instead, read the file in your InputScript and pass the file content to the RFC function via  a "table" parameter.