I can see so many Google results on a hanging quotacheck that I feel unable to solve this. It could be anything. Some issues with the file system for example. Named pipes for example. Maybe a mount that is going to a very slow external location. Whatsoever. Maybe you should consider to ask Plesk support whether they can dig into this deeper to figure out why it happens. However, I don't think it is a Plesk issue. Rather a general issue with the disk or OS.