I finally found the reason of the lock.
In outputfilter.pas, an unexpected exception is raised during the main Execute process. As all this part of the code is protected by a try/finally/end block, the code runs directly into the finally step. This one is supposed to clean up all the process, but apparently this is done here is an inconsistent state: so, the lock.
Anyway, this module (outputfilter.pas) is no more present in the trunk version. It seems that 'EnableNewExtTools' is now always set by default. So, no need to introduce an unnecessary bug report for the 1.2.4 version of Lazarus...