You’re right. Server error didn’t cause the problem, it’s the result of it. The only thing I can figure is that, my attempt to be doubly certain I got the DB backup caused the problem. I already had a successful backup via PHP MyAdmin. Then I tried the WP DB Backup plugin, which warned me it wasn’t compatible with WP 1.5 but I still suspect that I clicked on something I shouldn’t have and cause the plugin to corrupt something.
I certainly can delete plugins via FTP. I was interested to know if anyone else had successfully used that approach when admin panel is not accessible.