>If I define the Process class to use RedirectStandardError=False, I obtain an output message from WZZip.exe which is:
>
>"win32/windows system error (print.cpp@311): The handle is invalid."
>
>So, this makes it at least that the program doesn't hang queuing in memory all other processes waiting in pending.
>
>But, I cannot find why this cannot run and gives this handle is invalid message.

Do you have a reliable repro for the problem, or is it intermittent?

If the former, maybe the contents of one or more of the files you're zipping is causing problems. In that case you can use a binary-style search to narrow it down quickly i.e. try zipping half the files, if still bad try zipping half of those etc.

If it's intermittent - is there any chance that two WZZip instances could be running simultaneously? Some zip utilities create temp files. If the temp file names are not unique you could get a collision if two instances run at the same time.

Another possibility might be if you're making successive calls to WZZip back-to-back. WZZip may report that it has completed, and behind the scenes instructed the file system to delete temp file(s), but the file system may not have finished deleting temp files from the last run before the next one starts. ISTR in some cases zip utility temp files can be large, up to twice the uncompressed size of the files you're zipping. So, it could take a while to delete large temp file(s).

Finally, if temp files get large, could you be hitting disk space limitations, or space quotas that may have been set?