It would have been nice if you had stated you were using Italian as the language in the initial post.
I forgot to tell you this.....
...but I found the problem!!! If I login as admin in webmail using english all seems ok, if I try to do this with another language the problem remains.
Would be a translation problem?
I think I'll try to uninstall-reinstall with your new rpm.
Another question to you John: if I uninstuall the packages (I presume with yum remove _rpm_name_) and then reinstall will I lost the bookmarks and calendar entries?
Sorry for my terrible english but in english exam I took 18/30.... 
You shouldn't lose any data because that information is stored in the mysql db. I also don't think it's a translation issue. I set my preferred language to italian and french and I see the sidebar as well. One post from the horde archives mentioned someone had to change the sql backend in horde/conf.php to null, login and the sidebar worked, then changed it back to sql and the sidebar continued to work. But, there was no detail on what caused this and why.
I am loading a new test server to try to duplicate this. Did you upgrade to horde 3.1 from my horde 3.0 add-on rpms?
You may have to take this issue up on the horde list for final resolution.
John