Hello,
Please help. I just moved my friends website over to one&one and zenphoto stopped working. I am not sure what to do, but it seems like .httaccess is giving problems. Specifically, the previous web developer, (whom I am contacted, but had no response so far) made some changes by adding the following lines at the end. If I comment out the first rule, the script doesn't cause the 500 error. But doesn't work when I click any links in the gallery. ( I get page not found. If anyone knows, I would greatly appreciate your help. By the way, I have followed the steps in the 1&1 installation tutorial to the point, except like I said earlier, I was moving between the webservers, and not starting anew.

Catch-all - everything else gets handled in PHP for compatibility.

RewriteRule ^(education-cultural-civic|commercial|retreats-religious|residential|hospitality|meta-planning-urban-design|c)/+?([A-Za-z0-9_-]+){1,}/?$ index.php?album=$2&category=$1&project=1 [L,QSA]

RewriteRule ^(education-cultural-civic|commercial|retreats-religious|residential|hospitality|meta-planning-urban-design|regenerative-design)/?$ index.php?album=categories/$1&category=$1 [L,QSA]
RewriteRule ^(.*)/?$ index.php?album=$1 [L,QSA]

Thank you!
(upgrading to the latest version didn't help. - i have it currently at 1.3 and the upgrade in the admin area is not working. also, the duplication of themes not working either. there were no errors in the setup.

Those are rewrite rules that redirect something. Why those have been added you need to ask that web developer. I can tell you that those are neither Zenphoto rewrite rules nor would they be necessary.

So I recommend to delete the htaccess file and re-run setup to let Zenphoto create a new one. For the instruction see: http://www.zenphoto.org/news/installation-and-upgrading

If you wish to move a site (= its content) from one host to another please also read here:
http://www.zenphoto.org/news/troubleshooting-guide#troubleshooting-installation (linked within above page as well).