I had 3 members of my board take a trip to another country. One of them got in contact with me saying "Member 3" can't log in, it would log him out as soon as he logged in. The first thing I did was reset his password and tried to log in with his account and it was fine.
"Member 3" tried on all 3 laptops, they all had separate laptops. Well this was a bit of a "fun" trip so they didn't feel like giving me much info since they had other things on their mind plus there was 12 hour time difference.
I did find out other items when they were waiting at the airport.
The owner of the hotel had the same problem, but just gave up and never contacted us.
Member 1 & 2 could only get in with their laptops but they never signed out and cleared their cookies, when the others tried a different laptop they used a different browser. So basically if you logged in from another ISP and never logged out you were fine.
Member 3 was able to register a new user with no problems, but he couldn't log in with the new user.
Now I do run an adult board so I could understand blocking by the ISP but I figured that would be affect all 3 members, even the 2 that never logged out. And blocking wouldn't explain the success in registering a new user.