I've been installing WordPress via Web Platform Installer 4.5 with a pre-installed MySQL 5.6 on a Win 7 system (development machine). Ran into abovementioned error during installation process, reinstalled MySQL, failed with default server instance, added *additional* server instance with *new* user "root" --> worked!
*Same* procedure on a Windows Server 2008 R2 (productive machine, system restarts etc. all included) does *not* work: I am still running into the same error. I'm out of clues here. Googling reveals that problem in general is notorious since at least 2010. What's going wrong here, how can this happen?