This does not seem to be a Rebex issue. The server just seems to be reporting a misleading error code. To determine what is going on, please create a communication log - as described at
https://www.rebex.net/kb/logging/ - and post it here or mail it to us for analysis. It might also be useful to try running the equivalent SFTP code on a PC - most likely, the same error would occur if you tried the same action, and it might be easier to debug this on the PC than on an Android device.