After a few attempts I managed to create a working backup on the Joomla 1.5 Intranet with AkeebaBackup. Initially I saved a 5.5 GB backup to disk but that couldn't be restored successfully. So I had to login over a vpn connection to the website and create a new backup, leaving out the DocMan documents and PhocaGalleries.
I tried to upload that through FTP but that didn't work. FTP traffic was blocked on the firewall. So I downloaded that backup (186 MB) and managed to send that with Wetransfer.
I just restored the backup and created a new empty Joomla 2.5 website.