I just found this post b/c I have the same problem.
Host - 1and1 USA.
I found 2 fixes to this problem. For me, the php5 solution listed above worked. But I also found an odd suggestion from faq.1and1.com. I tried to add a custom error log to my php, which fixed the problem.
I created a file called errorinc.php which contained the script listed here
then included this in my wp-signup.php.
I *think* this works b/c it suppresses errors.
I then re-ran the wp-signup.php (from IE6) . It works, but slowly, and spits out a bunch of errors to the .csv file. If you would like, I can post the errors. It appears to be erroring at wp-includes/functions.php:254, wp-includes/plugin.php:52, and wp-signup.php:254, 364, 361, and 244.
A 3rd working solution is to copy the wp-signup.php into wp-admin/wp-signup.php, and edit the includes for the correct folder.
Hope this helps someone.