I have exactly the same problem with 1and1 webmail.
A main difference with the last version of cforms is the doctype declaration. Now we have an XHTML strict; previously none.
<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Strict//EN”
“https://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd”>
<html xmlns=”https://www.w3.org/1999/xhtml”>
