I am running V2.7.6 64-bits on four machines.
All three state an update is available and direct to V2.8.1.VC++64bits
When downloaded, it does not offer or perform any update but installs V2.8.1 in a new folder.
When run, V2.8.1 briefly opens and then disappears from the screen and can no longer be accessed and musrt be shut down through task manager.
The same happens on all three of my machines.
I have found no alternative but to uninstall V2.8.1 and continue with V2.7.6
Please make sure you never mix files belonging to different Avidemux versions in one install directory, especially never mix MinGW- and VC++-compiled binaries. Additionally, never run multiple instances of Avidemux concurrently.
Instead of 2.8.1, I would recommend using a recent 2.8.2 nightly build for the sake of fixed bugs and improved stability.
It is known that Windows Defender with "cloud protection" enabled freaks out (https://avidemux.org/smif/index.php/topic,19923.0.html) when confronted with the modular structure of Avidemux.