If you experience problems when migrating your website built with Joomla 1.5 from one server to another, check my hints. There is no guarantee, but it is worth trying anyway.
Changes in configuration.php
As I just spent hours - actually the whole night - on this stupid problem, I hope I can prevent others from running into it as well.
For migrating your joomla site from one server to another, there are several tutorials on the net:
http://peter.pbwebdev.com.au/joomla/16-development/42-migrating-joomla-websites-to-a-new-server.html
http://kb.siteground.com/article/How_to_transfer_Joomla_15_from_one_host_to_another.html
They, and the book I have, all say, you should change the entries in configuration.php locally on your pc and then upload it to the new server.
So I did - and got terrible error messages.
After trying many things, I again uploaded the whole joomla directory to the new server, in this case "forgetting" to change the configuration.php.
Before uploading it later, I checked the website - and it worked! Then I uploaded the file - and got the error messages again.
After uploading the old file again, I went to the backend and changed the settings in the Global Configurations - all went well.
Worth mentioning: I migrated from my development folder to my www folder on the same server, using the same sql database. Maybe you have to change these settings beforehand, if you switch databases as well - I don't know - but I recommend you try first with the old configuration.php.
SEF urls and .htaccess
If you are using SEF urls and mod_rewrite enabled, don't forget to put the htaccess.txt from the joomla installation package into the new root directory and rename it to .htaccess .
You don't know, what I am talking about? But you want those nice urls like I have as well? Check http://www.siteground.com/tutorials/joomla15/joomla_seo.htm
See also: Pech mit Google