I've got another couple of reports about this issue.
Everybody who experienced it so far is using a 64-bit OS.

A commenter on YouTube said he managed to solve it by installing the tweaker to a different path.

I, meanwhile, couldn't reproduce it, and I have no idea what causes the issue.
If you can reproduce it on your PC, please contact me. Thanks!

Update: please read about the issue and v4.2 here:
http://rammichael.com/7-taskbar-tweaker-v4-2

I've looked at it. The cause is that Windows 7 seems to special-case the behavior of the HSHELL_WINDOWACTIVATED message for the left mouse button.

For left click it goes like:
Press the mouse button -> Move/Release it -> HSHELL_WINDOWACTIVATED is sent.
For other buttons:
Press the mouse button -> HSHELL_WINDOWACTIVATED is sent.

So between the time you press the middle mouse button and the time thumbnails are shown, there considered to be no active window.

These shell message are quite a low level mechanism, and while it might be fixable, it doesn't worth the effort.

On Windows 8, it seem to work the first way for all mouse buttons.

I'm marking it as "not a bug". It's not a tweaker's bug after all.

Tried it now - installed clean Windows 7 64-bit, checked for updates, then installed the tweaker - it worked fine.
This seems to be a very random bug.

This is a different issue.
See my comment here:
http://rammichael.com/7-taskbar-tweaker/comment-page-12#comment-2540

Seems like a Windows 7 only issue - I couldn't reproduce it on Windows 8.
I'll look at it closer when I have time.

This was discussed in the blog comments:

http://rammichael.com/7-taskbar-tweaker/comment-page-13#comment-2569

Quoting myself:

"In fact, I’ve encountered this issue, but it happens so rarely (happened to me once or twice, literally) that it doesn’t really bother me.

I can’t do much with it as I can’t reproduce it.
Also, I’m not sure it’s the tweaker’s fault. If it happens often for you, you can experiment and see whether it happens without the tweaker running, or with it running with all options off."

Duplicate of:
http://tweaker.userecho.com/topic/142800-version-411-doesnt-work/

If you think you can reproduce it on your PC, and has some spare time to run a custom build or two, please contact me so I can solve this issue.

I've got another couple of reports about this issue.
Everybody who experienced it so far is using a 64-bit OS.

A commenter on YouTube said he managed to solve it by installing the tweaker to a different path.

I, meanwhile, couldn't reproduce it, and I have no idea what causes the issue.
If you can reproduce it on your PC, please contact me. Thanks!

Do you get any error message? Or is it just loading forever?
You might want to try Softpedia:
http://www.softpedia.com/get/Tweak/System-Tweak/7-Taskbar-Tweaker.shtml