DIAGNOSIS This is almost always one of two things: a worn-out or misaligned rubber sealing ring on the blade assembly, or the blade assembly itself isn't screwed on tightly enough.
FIX First, ensure the blade assembly is properly tightened. Take the jar off the base, hold it upside down, and use the provided wrench to give it a firm, final quarter-turn clockwise. Don't go crazy, but it needs to be snug. If it's already tight, then you need to replace the sealing ring or the entire blade assembly.
IF THAT DIDN'T WORK Check for a hairline crack in the plastic of the jar itself, especially around the threaded center where the blade screws in. Fill the jar with water and hold it over the sink, looking closely for a drip. A cracked jar can't be repaired and needs replacement.
HEADS UP Running the blender with a leak is bad news. Liquid getting into the motor base can cause the 'err' fault or a permanent short circuit. Stop using it until the leak is fixed.