Currently, 7u5 uninstall 6ux msi which will run customAction with jaureg, the jaureg will uninstall AU if condition met, then install 7u5.
Since the AU uninstall is not owned by 7u5 wrapper it is difficult to get correct handle to it. If AU uninstall does not release the msi mutex completely, it will collide with the 7u5 msi install and get the error 1500.

The previous approach was to try to catch the msi mutex and release it before running 7u5 msi. But, it does not seem to work well on some of the OS (particularly 64-bit win7).

The new approach of the fix would be to uninstall AU first from the wrapper and wait for it to finish, then run the 6ux uninstall and install 7u5.