Hi Martijn:
If you go to the controller's GUI to Commands > Upload File and manually trigger the TFTP Configuration backup, does the process complete as expected? If not, whatever's causing that problem needs to be addressed first.
When someone wants to do a config backup from the command line, they give a series of commands
transfer download datatype config
transfer download mode tftp
transfer download serverip x.x.x.x
transfer download path ./
transfer download filename myBackup.cfg
transfer download start
The Controller Config Backup background task is just the SNMP version of "transfer download filename " and "transfer download start". If these other required settings aren't already there, the task isn't going to set them (the humans have to). If there's a problem with writing out the file on the server, or packet routing issues to get to the server, the task isn't going to see those things, it just knows that the answer to "transfer download start" wasn't "Okay!"
As for watching this processing in logs, go to Administration > Logging, and set the logging level to Trace for the modules SNMP and System, and click Save. Then kick off your backup attempt, allowing it to fail. When it does, return to Administration > Logging, and click the Download button to retrieve the logs.zip file. The activity should be in the ncs-x-x.log file.
If you don't have the new Update-1 patch for Prime Infrastructure 1.3.0.20 (filename PI_1.3.0.20_Update_1-12.tar.gz) installed, I'd suggest you get it. While it's not going to specifically address this issue, there are a lot of really good fixes in it.
Release Notes for Update 1 for Cisco Prime Infrastructure 1.3.0.20
