QbsProjectManager: 'Clean' makes Creator unusable and blocked until manually hard-kill

Steps to reproduce:

• start Creator, enable QbsProjectManager plugin, restart Creator
• open qtcreator.qbs from 3.1 or master, wait until parsing finished
• have the project built already in the location where it would get build (or build it now)
• trigger the context menu on the project node and invoke 'Clean'

Current result:

• progress bar with 'Clean' appears, but get's stuck right after the start (waited for about 10min, but nothing happens)
• hitting the small 'x' at the end of the 'Clean' progress bar does nothing (or the 'Stop' button inside the 'Compile Output' pane), except that now Creator is completely blocked and unusable (cpu usage goes up to ~200%)
• Creator can't be closed except for a 'kill -9' call on the process