The problem is caused because 1&1 runs both PhP4 and PhP5 in parallel, and PhP4 is chosen by default.
1&1 FAQ says that code can be forced to run as PhP5 in one of two ways:
(1) if the php code itself has the extension .php5
(2) if the following line is added to the .htaccess file:
AddType x-mapp-php5 .php
When I put this line in the .htaccess for the root, the installation process starts correctly (though it generates errors further on - I will check whether additional htaccess files are needed in subdirectories or includes)
This seems to me of general interest - there must be quite a few servers running 2 versions of PhP.
A