Moved Gallery 1.5 to new server. Can't edit albums anymore.

I moved an installation of Gallery 1.5 to a new server. I changed the paths in the config, and set all files called album... and photo... permissions to 777, as well as the .users folder. I can view all the photos ok, but cannot edit any current albums or leave comments. If I create a new album, then everything works fine. (I also changed permissions to 777 on all photos in several albums, but this did not help.)The gallery has about 100 albums and several thousand photos.

Also, on first viewing an album, I get the error message "Warning: fopen(/...../photos.dat.0): failed to open stream: Permission denied in /...../gallery/platform/fs_unix.php on line 55

photos.dat.0 does not exist in any of my albums.
And when I click Refresh, the error message goes away, but I still cannot edit the albums or leave comments.

Can anyone help with this problem?
Thanks.