Quicktime opens my desktop folder

I can't open any MOV file on my iMac and when I click on QT application icon it opens a documents folder instead of launching QuickTime program.

What application is active and opening the "Documents" Finder window?

Double-clicking the QTX application icon should automatically open the QTX application which, in turn, should open a Finder widow instead of a media player window since the QTX app does not yet know which media file to open in the player window and wants you to select one. Since you indicate the QTX app is not opening when you double-click its icon, what app is opening up and, in turn, opening the "Documents" Finder window? (I.e., check the app name located to the right of the  icon in the top left-hand corner of the menu bar.) If the QTX app is indeed open, then your system is working normally. If another app is opening, then it sounds as if your MacOS install is corrupted and you should probably reinstall it. If another app is opening when you double-click on a media file (e.g., an MOV file), then check the "Open with ..." settings (in the "Info" window) associated with the media file class you double-clicked.

This is happening to me as a direct result of downloading Sierra today. It opens my "applications" folder,

This sounds normal. Have you navigated the "opened" folder to a location containing a video file and tried double clicking on it or selecting it and pressing the "Open" button?

and I cannot access QT player to do a screen record.

This doesn't sound normal and may indicate a corrupted macOS or QTX app install or a corrupted QTX PLIST file for the current user account. Have you tried reinstalling the macOS or opening the QTX player app under a different user account?

I've just noticed an odd problem with Quicktime. When I click on the app to open, it literally opens the desktop folder and not the Quicktime app. Furthermore, when I try to play a file through Quicktime it says that Quicktime cannot be opened. Essentially, I can't access Quicktime at all.

Does anyone have any idea as to why this may be happening?

Your question/issue description is somewhat ambiguous.

If you mean that when you double-click the QT X v10.4 Player icon a "Finder" window automatically opens, then it sounds as if everything is operating normally. That is, the application opens, sees that that you did not double-click a specific media file to be opened or don't have System Preferences set to automatically re-open any previously opened files in player windows, and, therefore, automatically opens a Finder ("Open File") window to allow you to navigate to/select a specific media file to be opened. (I.e., this is the default workflow for many Apple apps under the latest Mac operating systems.) In this case you can either double-click QT X compatible media files in the Finder to open them immediately in a QT X player window or turn off the System Preference in the "General" widow that closes currently open files when apps like QT X, Safari, Text Edit, etc. are closed/quit by the user.

On the other hand, if you mean something else here, please describe both your actions and their results in detail. It is possible that the the QT X Player and/or your operating system have become corrupted and might need to be re-installed.

I experience the same problem.

It started today after I installed new security update/patch for last iOS Serra.... I can't open any MOV file on my iMac and when I click on QT application icon it opens a documents folder instead of launching QuickTime program. I checked my system for viruses - everything is clear.

How can I repair this issue or re-instal QuickTime?

This is happening to me as a direct result of downloading Sierra today. It opens my "applications" folder, and I cannot access QT player to do a screen record.