Christian reported that 'quick clean' fails because the OPDA PDB file is in use. If this happens to you please check the following:
You should be running OPDA from a location on your hard disk that is outside the source code area. When you do a build of OP the Assistant is one of the applications that gets built so it needs to be overwritten during the build task.
Wherever you decide to locate the working copy of OPDA you should also copy the PDB file from the same debug build folder.
In a future release of OPDA I will ensure that an automatic update of OPDA copies both the EXE and the PDB as well so that we don't have this problem.