We are still waiting for log files to reproduce this problem. We are testing against IIS logs already but the import works. So anyone who experiences this problem: Please provide us a log file (with like 10 entries) that produced this error.
You can send the log to fabian [at] piwik.org or post it at dev.piwik.org.
You can send the log to fabian [at] piwik.org or post it at dev.piwik.org.