Are records still being imported? That is, is the 'number of records' increasing in Settings | Sources? If so, this error is most likely nothing to worry about. TMG Reporter may have encountered a record in your log files that it could not parse properly.
I also recommend upgrading to our latest Beta release if you haven't already - http://fastvue.co/TMGReporter21
In addition to some great new features it also includes performance improvements and bug fixes. The error you're seeing may be resolved by this build.
Sorry to hear the update did not resolve the issue for you.
Can you help us troubleshoot this by sending us a verbose diagnostic log? To do this: 1. Go to Settings | Diagnostic and set the logging level to 'Most' and click Save. Also note the location of the Dashboard.log file. 2. Go to Settings | Sources and click 'View all issues'. Then click the "Clear all issues' button in the top right. 3. Once the error re-occurs, send the Dashboard.log file to email@example.com so we can investigate this further.