I'm sure you know by now there's a special Stahlwille 8mm allen head type socket used to remove the fan bolt. I found that it also helps to have a LONG 3/8" breaker bar and just the socket to get the bolt loose.
The area on this bolt that the 8mm allen head goes into is made of very soft metal and it rounds off easily. I suspect a heavy-handed person paid it a visit some time in the past.
I went thru the same thing a few years ago and managed to use a pipe plug socket(male) to get the bolt out. SK makes these sockets. The bolt had been round off so I used one of these square pipe plug sockets. This is one of those times when a "square peg in a round hole" might work. I did for me.
Here's what they look like - 3/8s and 1/2 in. drive.