Hi Basj
I did as you tried, and while it didn't solve the problem, I think it might've pointed to where the problem is.
Immediately after reinstalling the center alignment was okay. I could switch between left and center alignment without any problems. I noticed that the setting in Start11 is simultaneously applied in the Windows taskbar settings, as shown in the screenshots below.


However as soon as I enable "Let Start11 enhance the taskbar" the Windows taskbar setting is set to left regardless of what setting is chosen in Start11.


To summarize: when Start11 is not enhancing the taskbar everything works fine. When Start11 is set to enhance the taskbar, the start button is left-aligned even though the rest of the icons on the taskbar are allowed to remain in the center.
Any other suggestions?