Before the migration, my site was running but it relied on XSLT. Apparently this needs to be enabled by the administrators since I was unable to enable it myself. http://us2.php.net/manual/en/xsl.installation.php
I'm aware that you're busy working out all the issues from the migration and that I'm only a free customer but I'm hoping very much that this can be resolved soon. (As is vidryfrx who is apparently experiencing the same problem.) I am optimistic that my link above may be helpful since it looks like the xsl extension wasn't installed by default.
Hmmm I also have started to encounter this on my free hosted site, like you this is my own code which relies on xslt a fair bit. Hopefully this is going to be resolved shortly.
I'm going to sweep through the free hosting servers in a few days to make sure they are all using the same php configuration, ensuring that the XSLT extension is available will be part of this.