This is not a big concern... easy fix. With version 2.5.2, there is a new folder that is supposed to be created on install at /images/dtregister. This where files are stored that you upload for email attachments. For one reason or another, this folder was not able to be created on your site automatically. Simply go to the Joomla /images directory and manually create the new folder "dtregister" (with no quotes of course). Make sure the folder is writable and you're all set.
I am upgrading to DT Register 2.5.9. I have created the directory images/dtregister and made it writeable, but I'm still getting the JFolder::create: Path not in open_basedir paths error.
Don - this is an old thread but there is at least one newer thread addressing this. Same issue... permissions on your site. Now you need these directories with 2.5.9: