Updated Flash Player won't run local Videos

This is most likely due to a security change in Flash Player 23 to prevent accessing local files.  There is a work-around. From the Release Notes Flash Player 23 AIR 23 :

Disabling local-with-filesystem access in Flash Player by default

Beginning with Flash Player 23, local-with-network permissions will now be applied to all local SWF content, regardless of the preference chosen at compile time.

Background:
When playing Flash (SWF) content from local filesystem, developers have historically been able to configure content to exclusively read from the filesystem, or communicate to the network. When this functionality was introduced over a decade ago, it enabled an interesting array of use-cases ranging from simple games to interactive kiosks. In context of modern web security, we believe that it is time to retire local filesystem functionality in the browser plugin. At the same time, Adobe AIR has been established as a robust, mature solution for delivering ActionScript-based content as a standalone application.

Vast majority of Flash Player users and content will be unaffected by this change. This change only impacts Flash content played from the local filesystem, using the browser. Flash content hosted on the internet and local webservers, as well as the Standalone Flash Player remains unaffected.If you are a user who requires this functionality, these files can be added to the list of Trusted Locations in Flash Player.

Workarounds for Legacy Content:
We highly recommend that you only circumvent these controls to enable content from sources that they trust.

For Individuals:

For Internet Explorer, Edge, Firefox, Opera and Safari:

On the affected system, go to the Flash Player Settings Manager:

• Mac: System Preferences > Flash Player

• Windows: Control Panel > Flash Player

Select the Advanced tab

In the Developer Tools section, click the Trusted Location Settings button

Click the "Add..." button and add relevant files and folders to the list

For Google Chrome (and similar PPAPI browsers):

Navigate to the Settings Manager page

Choose Edit Locations > Add Locations from the popup list

In the text field that appears, type or paste the file/folder path that you'd like to trust

Click the "Confirm" button

Note: Please be aware that the "Browse for files" and "Browse for folder" buttons do not function properly. You must manually type or copy/paste your path into the text field above the buttons to add the file or folder to the trusted list.

For System Administrators:
The legacy behavior can be restored by applying the EnableInsecureLocalWithFileSystem=1 flag to mms.cfg.