After starting one or more long running actions, user becomes blocked in given browser session. Logout/Login doesn't help. Logging in as same user in different browser allows user to interact w/ server again. UI becomes "unlocked" and responsive again once the long running action is finished.
Problem is more pronounced with Firefox and Chrome then with Safari. Didn't test on IE.
While server became very "sluggish" at some point due to number of running actions, our tests didn't confirm ability to completely bring down the server at this point.
The issue was not reproducible at all in Magnolia 5.0 using older version of Vaadin, which suggests issue is related to some change (probably introduction of locks) in Vaadin 7.1.
More details discussed/described at http://wiki.magnolia-cms.com/display/DEV/Concept+-+Long+running+actions+intermediate+solution