This is the second upgrade to RC1 that has had problems. Can you please try these things first? This looks like a bug.
(1) Please give us following information:
Joomla version : ?
JoomlaComment version : 4.0 RC1 (already know)
Upgraded from JoomlaComment: ?
PHP version : ?
MySQL version : ?
Joomla!, PHP and MySQL version can be found under Help > System Info in the Joomla backend. MySQL should just be listed as 'database version'
(2) Try clearing your Joomla cache first and also your browser cache, refresh the page and see if that fixed it
(3) I will assume the versions are OK and if (2) did not work, then it is probably a bug. Do me a favor so we can track this down:
(a) I assume your database prefix is the default of jos_ in this guide but it may be different on your site. Before uninstalling anything, in phpMyAdmin or whatever database frontend you use, export all the following tables:
jos_comment
jos_comment_captcha
jos_comment_joomvertising
jos_comment_setting
jos_comment_voting
Take jos_comment, the first one, and keep this - this is the comment backup!
(b) ZIP all the other tables (jos_comment_*) and send them to me/us to investigate - there is an E-Mail address "hidden" from spam in my signature.
(4) Now you can go and uninstall JoomlaComment. It doesn't matter if you leave uninstall complete mode to NO - if you do, just manually go into phpMyAdmin again and drop all those jos_comment* tables.
(5) Reinstall and clear caches, test. Meanwhile we will see if we can track the bug as soon as I can see if the tables are OK.