Open a RAR-archive in a lister and extract a file via d'n'd. When progress window for this operation appears, choose another file from the same archive and try to extract it same way. Nothing will happen until the previous process has been finished (so that's no multitasking!).
To reproduce be sure to use some hundred MB files to keep DO busy while you choose and try to extract the next file.
Maybe the archive is locked from first process, but there's no error message and under DO8 it was possible to extract 2+ files parallel from same archive.
The unrar.dll does not seem to be designed for multi-threaded use, and we have found it can cause significant problems if multi-threaded access is attempted - therefore the latest Opus RAR plugin blocks attempts to use it simultaneously.
BTW, there definitely are problems if Opus doesn't single-thread the RAR DLL. For example, changing to thumbnails view on a multi-CPU machine set to use more than one thumbnail thread would crash the thumbnail threads fairly quickly, apparently because unrar.dll is not thread-safe. If it worked for the odd file in the past you were probably just lucky with the timing. Having two or three thumbnail threads trying to read from the same RAR at once caused problems consistently.
