[TYPO3] language detection doesn't work anymore

[Frank Sonck]

Hello people,

I've got a problem that I can't get solved: on our (live) site (TYPO3 
4.1.7) the language detection (rlmp_language_detection) doesn't function 
anymore. We have Dutch as default and French as second language. But now, 
on a 'french' computer people see the default (dutch) version of the 
page. 
On another TYPO3 install (same server) this still works fine.
As far as I know nothing has changed with respect to language config in 
TS templates etc.

I have:

- deactivated realurl to see what happens: there is no language variable 
L in the URL
- upgraded the other TYPO3 install to version 4.1.7 also but language 
detection still works fine (of course)
- deleted and re-installed rlmp_language_detection
- upgraded static_info_tables
- removed cache in typo3conf

All this doesn't work. Does anyone have a clue what could be the problem 
of hints for how to look for a solution?

Thanx a lot for any help!

Kind greetings,
Frank Sonck