The "Middle click to mute" option requires Windows 11 version 22H2 or newer, and isn't supported for the old taskbar if a tool such as ExplorerPatcher or StartAllBack are used. If you think it should work, please create an issue here:
https://github.com/ramensoftware/windhawk-mods/issues

Regarding the "especially" word you used, what other issues are there? Please report any issue on GitHub with the relevant details.

Which version of StartAllBack is being used? Versions 3.8 and newer aren't supported, see:
https://tweaker.userecho.com/topics/1307-7-taskbar-tweaker-isnt-compatible-with-startallback-38

The solution for error 102 is to reinstall the tweaker, as the error message says. Please reinstall and let me know if the error persists.

Sure, a solution would be something along those lines, but there are several unknown variables, including when exactly to apply the new width, and how it's represented (e.g. in case of monitors with different scale settings). It was reported that restarting the tweaker doesn't make the taskbar narrow again, while I expected it to work as the last used width is stored in the settings. In short, it requires time and effort which I'm not able to invest currently.

I looked at the log, but it didn't yield the result that I hoped - quickly seeing the reason for the problem and coming up with a fix. Unfortunately, I think the easiest way for me to understand the problem is to get an environment that can be used to reproduce the issue. That requires time, and while I might have a look at at some point, for now it's not planned, sorry.

Well done. For Windows 11, I suggest to look at Windhawk. Most of the 7+ Taskbar Tweaker functionality was ported to Windows 11 as Windhawk mods (see details here). For example, the Taskbar Thumbnail Reorder mod's code can be useful for this task.

I'll be honest, this issue is unlikely to be ever addressed. This is not a simple task, Windows 10 is going to be unsupported soon, and 7+ Taskbar Tweaker is going to become irrelevant. Currently I focus on Windows 11 mods in Windhawk.

That's a question about Windhawk, not 7+ Taskbar Tweaker, but I'm glad that you found the solution.

Are you using ExplorerPatcher? What's your Windows 11 version?

Note that Windows 11 version 24H2 isn't supported by 7+ Taskbar Tweaker. More details can be found here:

https://ramensoftware.com/7-taskbar-tweaker-on-windows-11-with-windows-10s-taskbar