[TYPO3] Web and File modules not working in BE

[Rodrigo Cea]

The Web and File modules suddenly stopped working on a 1.5 year old 
site, using TYPO3 Ver. 3.7.0.
When I try to open "page", "view", etc., I get "Error! The page did not 
exist or was inaccessible. Reason: Segment "typo3" was not a keyword for 
a postVarSet as expected!"

The URL of the frame with the error is of the form: 
http://mysite.com/typo3/alt_mod_frameset.php?fW=245&nav=/typo3/mod%2Fweb%2F..%2F..%2Falt_db_navframe.php%3F&script=sysext%2Fcms%2Flayout%2Fdb_layout.php&id=

Whereas the User and Tools modules work, and the URL of their frames is 
of the form:
http://mysite.com/typo3/ext/beuser/mod/index.php?

The site itself is working fine, but I can't edit it and am starting to 
get nervous.

Any help is greatly appreciated.

Rodrigo