I want to say that they had servers seized running up to the 2004 Republican convention in Minnesota. However, I'm having a hard time finding news links about this.
I also went to check their wikipedia page to see if there was a history section, and Riseup doesn't seem to have one.
If I remember correctly... it was because they were offering free hosting to political groups and someone was using it to host bomb plans without them knowing.
I'm sure the time I'm thinking of had to have been before 2004 because I remember not even having a drivers license at the time.
I also went to check their wikipedia page to see if there was a history section, and Riseup doesn't seem to have one.
Now I'm going to go soothe my paranoia.....