RobotStudio event

6006 IMSTP Error

We have 3 ABB robot 2 welders 1 gripper. One of the welders keep coming up with an Joint 5 speed violation Erros on the pendant and on the HMI it says IMSTP error. It happends about 100 times in 24h shift. I've reduced speed to 75% still does it...What can it be?