Wouldn't it be more logical for Rainmeter to provide such functionality?
Anyway, what you want is the "Reserve empty space on taskbar" option, but with a customizable width?

Great :)
Also, one more hotkey Windows provides by default is Shift + Left click.

Works correctly for me on Windows 7 x64:
http://imgur.com/a/zShXk

Note that this option doesn't have effect on a vertical taskbar.

Should be fixed in v4.4.5.5 beta.

Currently, Windows 8.1 Update 1 (and the equivalent server version) are not supported. I believe you're using some sort of leaked/beta version?

Seems like a duplicate of:
http://tweaker.userecho.com/topic/382920-after-wak...

Please, provide a crash report.
How do I find event logs when a program crashes?

Please see my reply here:
http://tweaker.userecho.com/topic/308231-cycle-bet...

Yes, it works only for the main taskbar.
I believe that adding support for the secondary taskbars isn't worth the complexity.
First, the secondary taskbars have a different mechanism that limits the width.
Second, the tweaker will have to remember the sizes of all available taskbars.

I could reproduce it on an RTL system.
Seems like a bug of Windows or explorer. The issues happens without the tweaker as well.

I can't reproduce the issue.