Installation Manager can't connect to repository

I'm trying to install RTC 3.0 iFix 1 under Windows 7 64-bit. This is currently failing, apparently because the Installation Manager cannot make a connection to the repository location specified in install-server.xml:
https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.config
Opening this URL from the browser works without problem (after authentication).
When testing the connection to the repository from within IM the test fails due to a timeout, see below (sorry for the German bits in there). Anybody have an idea how to work around this?

Error details:

"Die folgenden Repositorys sind nicht verbunden:

-https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.config

Wiederaufnehmbarer Download fehlgeschlagen: https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.xml.
https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.xml wird heruntergeladen.
'https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.xml' wird nach 'C:\Users\bolik\AppData\Local\Temp\cicdip_bolik\1304351913569\nf\cicURLLrepository.xml6560968897693330862xml' heruntergeladen.
Operation timed out: connect:could be due to invalid address
Antwortzeit 00:21.02.
Wiederaufnehmbarer Download fehlgeschlagen: https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.config.
https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.config wird heruntergeladen.
'https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.config' wird nach 'C:\Users\bolik\AppData\Local\Temp\cicdip_bolik\1304351934826\nf\cicURLLrepository.config3845442175651820949config' heruntergeladen.
Operation timed out: connect:could be due to invalid address
Antwortzeit 00:21.01.
Das Repository des Typs CompositeRepository kann nicht an der Position https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository erstellt werden.
Wiederaufnehmbarer Download fehlgeschlagen: https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.xml.
https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.xml wird heruntergeladen.
'https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.xml' wird nach 'C:\Users\bolik\AppData\Local\Temp\cicdip_bolik\1304351913569\nf\cicURLLrepository.xml6560968897693330862xml' heruntergeladen.
Operation timed out: connect:could be due to invalid address
Antwortzeit 00:21.02.
Wiederaufnehmbarer Download fehlgeschlagen: https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.config.
https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.config wird heruntergeladen.
'https://jazz.net/downloads/rational-team-concert/3.0/iFix1/install-repository/rtc-offerings/repository/repository.config' wird nach 'C:\Users\bolik\AppData\Local\Temp\cicdip_bolik\1304351934826\nf\cicURLLrepository.config3845442175651820949config' heruntergeladen.
Operation timed out: connect:could be due to invalid address
Antwortzeit 00:21.01.
"