I may be having the same trouble as this http://drupal.org/node/350565
My web sites have become "read-only." Doing ANYTHING at them which should push some info in the database gives only a page refresh, with no feedback.
I upgraded from various versions of 5 to 5.14, things seemed fine, I dumped all the databases as a backup (numerous times along the process).
Some of the sites I updated to 6.8 and ran in a multisite configuration. Those sites seemed to work fine and the upgrades in Drupal 6 were nice. I try to run as lean sites as possible without any additional modules: KISS.
I had two sites with some modules that have not yet been updated for Drupal 6, so I decided to to a multisite config for them. These two Drupal 5 sites never recovered. I feel certain the databases are not corrupt since I can load them in MAMP and they seem fine. But when everything is set up at my host GoDaddy.com, the sites have become "read-only." I can access the sites, read the content, but when I try to log in, the login page merely refreshes.
So after this pain (and days of trying various things to work), I decided to put one of them in to Drupal 6.8.
At first everything seemed fine. I could log in, and change any settings, all my other Drupal 6.8 sites seemed fine too. I logged out of all but one Drupal 6.8 site and went to bed in the wee hours.