So is there any headway with this from Apple?
This is a user-to-user forum. If you want to report the problem to Apple formally, then use the Product Feedback or Apple Developer Bug Reporter forms as needed. While Apple may monitor these forums on a periodic basis, it rarely participates in these forums once they are up and running on their own.
The problem as stated by the original poster (OP), as well as most follow up posters, provides insufficient information to provide a specific solution to the stated "issue." That is to say, there could be several different issues at work here for different users. For instance, the above users indicated the problem began following the Mac OS X v10.11.5 upgrade but only one individual indicated that the upgrade was from v10.11.3 to v10.11.5 version of the Mac OS. Thus, if any of the other users were upgrading from a pre-Mavericks version of Mac OS X to El Capitan for the first time, then their problem could be associated with the codecs used to create their files. So the first step in solving the problems here is to determine if the problem is associated with the files themselves, the operating system (embedded QT structures), the QT Player GUI in use, support files associated with the QT Player GUI in use, a system security issue, etc.
Since I have none of the issues mentioned, it appears that only a small percentage of users are affected, and the issues appear to vary widely, I suspect the problem for most of the users here is some sort of corruption introduced during the upgrade (download and installation) process. While rare, this has happened to me on more than occasion over the last several years since Apple switched to software distribution via the internet. In cases where I had a problem, running the installer a second time corrected the problem. If you are uncomfortable performing specific diagnostic actions to locate and fix specific issues, you might try this "general solution" approach.
For more detail oriented users, the following addition information might be of interest...
1. With regard to the use of alternative user accounts mentioned above, this tests whether or not the system and app is working correctly. E.g., if the QT Player works correctly under one user account but not another, then the system/app is probably working correctly but a support file associated with the specific user account has become corrupted. On the other hand, if the problem exists under all user accounts, then the problem is global in nature and is most likely in the app or operating system.
2. If the problem is global, then the next step is to determine if the issue is associated with the app itself or your OS. In this case, you might try reinstalling the app (and all of its associated files) to see if this fixes your issue. If this fixes the problem, then you are done. If not, then you can still try reinstalling the OS as previously mentioned. Unfortunately, the QT X Player is not available as an independent download since it can only be installed on a matching compatible OS version. Further, since many post-Mountain Lion users were actually deleting the QT X GUI (by accident or on purpose) and, thus, self-inflicting QT problems, Apple has currently trapped the OS to prevent user removal and, once again, we are left with using TimeMachine to restore the app or re-installing the entire OS if that doesn't work.
3. Security problems can be more difficult to analyze. Basically they can be associated with the creation of a file using older software apps or hardware devices that create a container file that more recent software/hardware versions now consider a security risk that "could" allow your system to be infected with malware. In most cases there's nothing really wrong with your files and, if you have the ability to do so, simply copying the data from the old file container to a new one will satisfy error trapping and allow the new file to play correctly.
4. If the original files employ legacy or third-part codecs that are no longer compatible with post-Mountain Lion QT X AV structure embeds, then the normal solution is to either transcode content to currently supported compression formats or use a media player that remains playback compatible with your specific content. Luckily, most Mavericks, Yosemite, and El Capitan users should already be familiar with this issue and have taken steps to remedy their individual problems by this time and it is only included here for those switching from older operating systems for the first time.
I personally recommend keeping an external hard drive docking station on hand for various troubleshooting workflows. (I.e., such devices are available from Amazon for as little as $30.) Using such devices can allow a user to "drop in" any available bare SATA HDD and get it up and running in moments with alternate installations of the same or different Mac X operating systems for analysis and testing. I also use it to switch between multi-TB data HDDs to access project files not kept stored on RAID. In any case, this workflow makes it easy to cross connect files and apps under the same or different operating systems to logically troubleshoot issues—particularly after operating system updates.
