Unlike previous dev builds, that could interface via a specific plugin (MechJebMenuToolbar.dll, by sarbian) to the toolbar plugin (by blizzy78) so to also allow buttons to be binded to the toolbar, version 2.1.1.01 only supports its native lateral tag with mini buttons for each module.
Reinstalling a previous dev build, like e.g. 2.1.0.0-139, in lieu of the new release version, eliminates the issue.

Did you try to re download MechJebMenuToolbar ? I uploaded a new version after I made the MJ change that broke it.

@sarbian : thanks.
No, I did not spot there was some new version of your plugin that could be in relation to the new version 2.1.1. - but I probably did not even update to the last version of your plugin, you said compatible with dev build 139.
I can't actually state a specific version, but the date with the plugin is 18/12/13, 22.04. And that works on my install with MJ dev build 139.