After succesful installation Symantec Installation Manager 7.5.21 displayed a new version available, just a regular upgrade. After upgrading SIM went into crazy loop showing the following message:
"Another installation is requiring a reboot to complete changes to the system. You must reboot your computer before you can use Symantec Installation Manager. then Launch Symantec Installation Manager again."
Sever (Win 2008R2) has been rebooted at least 4 times until the following solution was discovered:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\PendingFileRenameOperations (key renamed, not cleared or removed, just renamed)
There is corresponding MS kb article available: