If I click on an album link I get a blank page (hydra-glide.net/cpg) or HTTP 500 error, but only as guest. If I am logged in there is no problem. How can I resolve this?
Start by disabling any plugins you may have running. If that fails to fix the problem you will need to check the servers error logs. A blank screen or 500 error will get logged with all the details of what is going wrong. It is obfuscated to the front end for security reasons. If you can find that error it will make fixing the problem a lot easier.
You are using a theme derived from an old copy of the curve theme. You will have to fix some incompatibilities with CPG 1.6.x
In the <theme>/theme.php file, replace all mysql_xxx_xxx with cpg_db_xxx_xxx.
That sounds very logical, thank you.