It might be an issue with the RutOS firmware. What version is your device using? Could you open the OVPN client's configuration file with any text editor and delete the line below, if it is present in the file and if the original configuration uses tap setting, save it and try uploading again:
If that does not help, I would like you to attach a troubleshoot file to your question. First, replicate the issue by uploading the file and attempting connection, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there.