I'm having a bit of a similar issue.
Everything had been working fine, I was on version 0.7.0. I started getting the following error on ALL nzb's from NZBMatrix:
» URL Fetching failed; (DENIED_INVALID_CREDENTIALS)
I did some googling, and saw that I needed to update my install of Sabnzbd. I did so, bringing me to 0.7.1. This fixed the issue, I no longer got that error, and my downloads resumed without a problem. a few days later, I saw there was another update, this time to 0.7.2 for Sabnzbd, and thinking I'd stay ahead of the curve, I installed the new update. Now, I'm back to the same error again. Denied Invalid Credentials...I keep searching for reasons of this, but they're all telling me to update to 0.7.1.
You can see the spot I'm in with that, as I'm already PAST 0.7.1
I see plenty of posts on nzbmatrix about invalid logins, I see nothing about the Invalid Credentials for the most recent version of Sabnzbd. I'm looking through my log files currently, and I'm not seeing anything there that would really help either.
Any help on this would be greatly appreciated. I'm still fairly new to all of this, and I'm trying to figure my way around. Even a point in the right direction would be wonderful at this time.