Older Versions of Quicktime

Analyze your video file to determine why it shows just a white screen, and perhaps then something can be done about it, based on these findings.

Invisor Lite software (or any of the other apps based on MediaInfo). Invisor displays technical information about your video, audio and photo files. Video codec used would be very interesting here, as well as other details that may be unusual (Extended tab).

You have restarted the computer?

Did you do any other software updates recently? We haven't been given much to work with but there has to be some reason it suddenly stopped working. I can only make suggestion of things you can try because I still run Mavericks on my computer and QT still works. I don't know if some security update was recently released and that locked off some component. I presume you have done standard things such as restarting the computer?

I'd recommend VLC except a big negative aspect of it on a Mac is the lack of multiple windows unless you're into generating clones with terminal commands.

Realize at some stage any old software will become more and more buggy unless it receives upgrades for newer operating system versions. I think QT 7.6.6 dates from Snow Leopard days and I am surprised that is is still working at all in Mojave, particularly with all the system integrity protection stuff they have been implementing.

I just checked for QT766 and it is listed as a 32 bit app. That means that possibly as soon as the next major macOS version is released it will no longer work anyway since only 64 bit apps may be supported. Either that or not upgrade macOS and don't buy any brand new Macs when it is released.

Apple products feedback links - http://www.apple.com/feedback/

???? I am not sure to what you are referring! All I did was post a link so you could send feedback to Apple about Quicktime features.

Have you tried using it in a different user account?

I would think you do not want any system updates. I am guessing you had your computer set to automatically update and something was automatically applied in the name of security that broke it. Since Apple dropped support for that version years ago nobody on the mother ship has noticed it or even cared if they did.