[TYPO3] Extension manager broken? Empty list...

[E.Hermanns]

On my localhost, i've some typo3-sites installed.

One of them seems to have a problem with the extension manager: when i 
choose "Available Extensions to install", the list is empty (Not even 
the core extensions are displayed). It's the same thing when i choose 
"Import extensions from online repository" and try to look up e.g. 
"login" or "register": the list stays empty, which is not the case with 
my other typo3-sites.

I checked for read/write-permisions on the involved directories etc, but 
found no problem.

Any ideas?