$lang set to "C" on login page

[phm]

I have a strange issue. On the login page, the $lang parameter is set to "C". In debug mode, I get these errors:

Warning: include_once([...]/admin/lang/C.php) [function.include-once]: failed to open stream: No such file or directory in [...]/admin/inc/common.php on line 172

Warning: include_once() [function.include]: Failed opening '[...]/admin/lang/C.php' for inclusion (include_path='.:/usr/share/php:/usr/share/pear') in [...]/admin/inc/common.php on line 172

Warning: array_key_exists() expects parameter 2 to be array, null given in [...]/admin/inc/basic.php on line 604

(replaced path to site with [...])

Something goes wrong with the language fallback in i18n() but I can't figure out what.

EDIT: on my local production website this is not happening. The only difference is that the online version resides in a /beta/ subdirectory, but I can't see how that could have something to do with it?

[shawn_a]

what version of PHP and GS
what does health check say.

[phm]

what version of PHP and GS
what does health check say.

GS 3.1.2
PHP Version 5.3.3-7+squeeze13
Health checks: everything OK.

[Carlos]

Any plugins installed? Language file(s)?
Is it your first GS install in that server? (that is, had GS worked before for you on that webhost?)