The forums are the user support forums, not the Plesk come and help you forums. If you want Plesk support, they require that you pay them for it. That's why they set up the user support forums. Here, you post your problem, and hopefully someone else has already gone through it, found the fix (or paid for it), and is willing to help you out.
Unfortunately, I haven't run into this. I do my cron work from webmin or console, instead of through Plesk, so I don't have the experience of running into this particular problem. However, I am still working on a few other issues that Plesk is having, that I'm trying to work out, or get worked out, to get some things fixed for me, and some others, so that everything works right...