After several tries (the website crashed, the pc crashed, wrong settings used),
the forum is completely downloaded. 
I'm VERY happy that the program did the job!

HTTrack Website Copier/3.49-2 mirror complete in 3 days, 12 hours 50 minutes 5
seconds : 940574 links scanned, 929416 files written (70477747191 bytes
overall) [21440292622 bytes received at 70202 bytes/sec], 58192902235 bytes
transferred using HTTP compression in 794792 files, ratio 15%, 2.5 requests
per connection

(4512 errors, 7177 warnings, 0 messages)