More for the topic......
I set up another 5.6 box a few days ago. After default install, I added smtp mail front with rbl support, and IMP/horde 3.2.1. I then installed U4. This box has probems with PPTP on the first attempt to another 5.6 box. It worked ok to older SME 5.1.2 just fine.
I experimented by adding the preU4 pptp custom template (10masq_pptp which is blank 0k) patch, expand template, reboot.
(don't forget to 'service pptpd stop') before you expand a template.
After reboot, remove the custom template, expand template, reboot......no more error 619 and in the very short term, PPTP seems OK between 5.6 and 5.6 using XP pro.
Hope this helps, or if it does not help please post it.
RS