Thanks, I could manage the update.
MC now behaves differently, but still doesn't work.
I have assigned "click" to "context menu" and "click and hold" to "toggle".
Now the behaviour is not always the same.
Sometimes (rarely), "click and hold" really select a file, but only if no other file has been selected already.
Most of the time, it shows the context menu (plus, the selected file is checkmarked, but as soon as I close the context menu, the checkmark is gone).
Sometimes, click an hold doesn not show the context menu (which would be the right thing), and shows a checkmark immediately after the mouse-down event; but as soon as I release the mouse button, the checkmark is gone.
BTW, something off-topic, but maybe of interest to you: The Multi-Update seems to have a small glitch. Nothing serious, but it can be confusing:
I had enabled "update for beta versions" and then used Help/Check for Updates to initiate the update. MC was shut down (as expected), but then, the Updater complained that MC would still be running. I checked using Process Explorer, but no MC process, so I told the Updater to go on. It went through the update without error message, but then finished without offering to start MC. When I started MC manually, I still had the old version. I then did exactly the same procedure again, and now it worked and I got the update.