I found a tiny bug sharing account with forum.
1. Register an account at forum page with a complex password like 'h,17w~_,6H\h?mB&p)&o;GI[Y"hk<.
2. Enter the password at https://adblockplus.org/?a=login
3. The result is "Illegal password" error.
Probably this problem is caused by the deference of password limitation.
Another password like kRkcqMOOb2WmsVvS1GqOAdgVStlzFn passes the authentication.
It's not unusual case for me... but would be hard to find the reason for beginners.