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?

The tweaker should remember the width upon restart. What OS are you using?
Also, please check whether the following registry value is being created:
last_taskbar_width (DWORD) in:
HKEY_CURRENT_USER\Software\7 Taskbar Tweaker

There's no registry option for this tweak, as far as I know.

Duplicate:
http://tweaker.userecho.com/topic/145644-taskbar-i...

The last time I've checked, UltraMon used it's own implementation of the taskbar, and there were no grouping or combining.