I can't reproduce it.
What is your Windows version? 32/64-bit? What is the version of 7+ Taskbar Tweaker?
Also, posting the tweaker's options and the taskbar's properties might help reproduce the issue.

If you're using the portable version, then the last_taskbar_width value should reside in the .ini file, not in the registry. How come that you're seeing it in the registry? Did you have the non-portable version installed before?

I understand. I guess that it's a matter of preference - some will find it more convenient, some won't.
The tweaker doesn't provide this kind of fine-tuning, it just uses the system volume control. If you want more customization, perhaps using a specialized tool such as Volumouse is the way to go.

Well, 7+ Taskbar Tweaker uses the system mouse control app.
If I understand it correctly, you want the scrolling to accelerate the faster you scroll, right?
Instead of tweaking the volume control, you can change the behavior of the mouse wheel globally.

This bug report is about Windows 8.1. The 107 error occurs in Windows 8.1 update 1.
Relevant bug report:
http://tweaker.userecho.com/topic/405411-could-not...
More information:
7+ Taskbar Tweaker and Windows 8.1 Update 1

I do have two keyboard languages. I know that Alt+Shift works, but what does it have to do with a mouse click?

By default Windows uses Left click + alt + shift.

I'm not aware of that hotkey, and I can't get it working. Left click where, on the taskbar?

The first alpha version is available.
More information: 7+ Taskbar Tweaker for Windows 8.1 Update 1: early alpha version

What error reporting are you talking about? How can I reproduce the issue?