> Why is produced this error?
This is because you have asked for a fillet radius that is larger than the curvature of your objects. This is a general problem with operations that involve offsets. Here is an example:
In fillet, the fillet radius is like the distance of the line that I show above. By asking for a radius that is larger than the curvature of the surfaces, you were producing the situation to the right, where the pieces of the fillet surface would be running into each other.
MoI tries to compensate when it detects this situation, and attempts to trim out the messedup area and fit a smooth approximation inside of there, but often times the approximation might not work very well, this is what has happened in your case.
Your radius of 0.5 works properly because it is like the situation on the left in the example above  in this case your radius fits completely within the curvature so it doesn't produce the runningintoitself situation.
In general the fillets will work better if you avoid the situation to the right and ask for a distance that fits within the curvature of the objects.
 Michael
