Thanks,
I remember that this happened after I've installed one of the plugins but I've isntalled a lot of them so I don't remember the exact one.
However, I do not have /plugins/upgrade folder, I have only /plugins folder.
Here are the contants of /plugins:
acl authldap authpgsql cloud indexmenu nspages safefnrecode styling
action.php authmysql auth.php config info popularity searchstats syntax.php
admin.php authorstats authplain extension move remote.php simplenavi usermanager
authad authpdo ckgedit index.html npd revert stats wrap
Maybe the problem is because I miss the folder that you have mentioned?