visser.labs – that is not the problem that I’m encountering or that most others are describing here. The problem that is described is quite different (404 errors and such), and the permissions are already set as defined (755).
However, I don’t know what the problem is. I just went through the trouble of duplicating my WP 2.7 site in its entirety (manually hacking the duplicated database to point to the new URL in a whole bunch of places, etc – took longer than I thought to copy the site to a new URL). For some reason, NextGEN Gallery simply refused to see the images at all, and also refused to let me edit its configuration options. So I reset them, and it began working fine in 2.7. I never could figure out what database entry was stopping it from working.
Then I upgraded to 2.8, to get a non-working site to play with and link to. Wouldn’t you know it, the darn thing WORKS! I have no idea why it works when my main site fails, but it does. Perhaps resetting the NextGEN Gallery settings and manually reconfiguring before the update somehow did the trick, but I don’t get it. It doesn’t make any sense to me, and I’m not sure I want to take the time to upgrade my live site again only to have to restore it again, which is somewhat time consuming.
Right now, it looks like resetting Gallery’s settings to the default before upgrading may be the solution. I don’t know for sure that it’s what solved my problem though, nor do I know if it’ll work if you reset the config after upgrading. In either case, it doesn’t delete the galleries themselves, and only requires you to reconfigure the settings pages.