I appologize that this is going to be vague, but all upgrades have gone without a problem until now. When upgrading from 2.2.16 to 2.2.17 the upgrade failed when it attempted to do something with the spamassassin directory (directory did not exist) and then failed with the dreaded "The PHP binary coming with ISPConfig does not work properly on your system! The installation routine stops here!" error. Which makes sense since I think PHP installs after spamassassin. Sorry this is so vague but when that happened I paniced without writing down the details and rolled back to 2.2.16 since this is a production server.