Point 7.4 SP6 Installation Error

A few days ago, I installed Point 7.4 Service Pack (SP) 6, and everything seems to be going fine.
However, I had been encountering some errors with its installation lately. As of today, everyone who tries to install Point or SP6 get this error:
"This installation package could not be opened. Contact the application vendor to verify that this is a valid Windows Installer package."
Because of this, I've decided to revert back to SP5. I've tried to reinstall SP6 using both the cached and the non-cached install options, but the same error still comes up with every attempt.
I even observed that the error pops up immediately even before the download starts when I attempted the non-cached install. Has anybody encountered a similar problem with mine?
I don't understand what could have gone wrong with the install package during the weekend because the other machines that were upgraded to SP6 are still working just fine.
Thanks in advance to those who can help.
