I am suddenly seeing this error and having other theme issues on different themes created with Artisteer. I know how to work with the tpl.php files and the css, etc. I just tried Artisteer to speed things up and then I would tweak the settings myself.
Another site where I see this error, is also not getting any of my themes styling. Strangely, I don't remember changing anything for that site to suddenly start having problems with the themes. Other than upgrading to Drupal 7.20 from 7.19. However, the error didn't show up right after that.
I don't know if this relates to another error I am seeing... so this could be off topic (unless someone things otherwise). I am getting memory errors, when performing certain operations, including doing updates. My web hosting provider said that maybe I need something more than shared hosting. Of course, for that one particular site, all was working well with it. The hosting provider set the php memory at 64M. Again, I don't know how that would relate to a site not presenting the theme I created, suddenly and seemingly without any changes being done.
Thanks in advance for any tips,