Yeah, I've been having issues with Cron on my server as well (playing with my stock)...I had to use /usr/local/bin/php for my php command. I also had to change the include("../../SSI.php") line to have the full path to the SSI.php, as the include-path apparently treated the . directory as the /usr/local/bin, not as the directory of the php file I was running.
So I'd suggest the command be the following:
/usr/local/bin/php -q /home/mortgago/domains/[your domain]/public_html/Sources/shop/dointerest.php.
Mind, the part after the -q should be the actual path to your dointerest.php. I'm assuming that the path above is correct, based on the errors you were showing.
Also, based on those, double check the dointerest.php file...that first error looks like you might be missing a '<' at the very first character of the file, somehow.
EDIT: Missed that daniel15 suggested the exact same thing...oops. Sorry for the duplicate info.