Class 'i18n_string_textgroup_default' not found [NEED CACHE CLEAN]

Possibly you need a cache refresh.

Indeed. Clearing the cache also works if you see this error after upgrading to RC1:
Fatal error: Class 'i18n_string_object_wrapper' not found in sites/all/modules/i18n/i18n_menu/i18n_menu.inc on line 31

I'm getting the same similar error, however it's on a different line.
Clearing the cache didn't fix the issue, nor did running update.php.

Fatal error: Class 'i18n_string_object_wrapper' not found in /home/testuser1/testsite1/sites/all/modules/i18n/i18n_menu/i18n_menu.inc on line 31

Well it's the same error/line I posted in #3, I added it here because I got it together with the OP's error message, and the same solution applied... At least, I haven't seen the error since clearing all caches.

oops my bad! didn't notice the line number on your post!
I've tried every way to clear the cache and I'm still getting the error.
What's odd is that sub-sites under this install don't get the error.

After having upgraded from 7.x-1.0-beta6 to 7.x-1.0-rc1, I see the error as soon as I open the site in order to make the log-in.
Fatal error: Class 'i18n_string_textgroup_default' not found in /home/test/docs/sites/all/modules/i18n/i18n_string/i18n_string.module on line 371

The i18n_string_textgroup_default error is solved by clearing the cache (#1, 2).

I posted the i18n_string_object_wrapper error here as well because it appeared to me to be related, and fixed by clearing the cache as well.

@Jean: if you are sure clearing the cache does not solve the i18n_string_object_wrapper error for you it's probably better to open a separate issue for that.

Marking the original error notice as fixed again.

Sorry, I had to re-open this. The solutions to clear the cache do not work in some cases, as this problem can prevent people from logging into their site. I am just experiencing this:

PHP Fatal error:  Class 'i18n_string_textgroup_default' not found in /var/www/drupal/7.x_sites/all/modules/i18n/i18n_string/i18n_string.module on line 315, referer: http://XXX.com/user

It is not really a clean work-around to encourage users to start modifying database tables by hand, so I do not see any solution at the moment for this when someone cannot log in to clear the cache. Is downgrading to a rc an option?

Cheers, Lars.

Solution is cleaning the cache.

If upgrading your site without drush or an admin logged in, sorry, bad luck. But this is not solvable by a module.

Editing out file path by user request.

This problem can also occur if your code expects there to be more than one language, and there is not. For example, my site was working fine but my simpletest was throwing this error, because in the context of my test, only the default 'en' language was defined.