Redmine

You write you are using Apache, I'd wager with passenger? Have a read at the user-switching security feature of passenger (http://www.modrails.com/documentation/Users%20guide%20Apache.html#user_switching), it might be that redmine runs as user nobody in the end. You can also check that by ps aux | grep -i redmine (or equivalent) after having loaded a page in redmine.

This is the output from # ps -aux|grep redmine:
www 71176 0.3 4.4 77260 34396 ?? S Wed01PM 1:04.56 /usr/local/bin/ruby /var/www/redmine/public/dispatch.fcgi

So it runs with www user, which should have access to the repository. I checked the log directory, specially the production.log after trying to access the repository and here is the last row:
Processing RepositoriesController#show .... [GET]
Parameters: {"action"=>"show", "id"=>"zzz", "controller"=>"repositories"}
Rendering template within layouts/base
Completed in 2332ms (View: 1296, DB: 128) | 500 Internal Server Error

Do you know any way to get detailed info about which svn command is executed and with which user?
Thnaks!

Ah, you have fcgi, not passenger.

You could try toggling the values for config.action_controller.consider_all_requests_local and config.action_controller.perform_caching in $REDMINE/config/environments/production.rb (lines 12-13) and restart the fcgi/apache, that should give you more verbosity (it will slow down redmine execution too as it disables some caching, but you can switch it back once it works), maybe you will find more of what you're looking for there.

Finally I found the problem. After replacing mod_fastcgi with mod_fcgid everything start working just fine! I have to mention that I had similar problems with mod_fcgid, on some projects with a lot of files when I click on repository tab it loads too slow (probably because the number of files) and after 40 seconds stopped with "Internal Server Error". I solved the problem changing FcgidIOTimeout from 40 to 620. Probably the time out was the problem on mod_fastcgi but I'm not sure about that, mod_fcgid works well and I'm not going to replace it for now. I never worked with ruby applications before but have to mention that in my opinion they are heavier than php applications for example so if you plan to work with ruby apps - buy contemporary hardware.

You can stop redmine from reloading new changesets on loading the repository tab, but you need to have a cron or a post-commit hook to load those changesets instead, see the FAQ for more info.