The co-parenting relationship is one that is sometimes difficult to navigate. This type of parenting plan is based on mutual respect and a willingness to work as a team for the good of the children. When both parents aren't doing this, things can deteriorate quickly.
One issue that makes this situation much more challenging is one parent is harassing the other parent. This behavior is unacceptable and must be stopped immediately. There are several ways that you can address the matter.
The parenting plan should be the first line of defense. It should outline what type of behavior and communication are acceptable in the situation. Both parents should avoid speaking negatively of the other. The children should be allowed to be kids and should never be used as messengers.
When harassment does occur, keep records of it. You might need to bring the situation up to your attorney to determine what type of action is necessary to address it. There is a chance that you may have to head to court to get the matter taken care of if it is impacting your children.
In severe cases, you might need to move toward only using communication methods that are recorded and monitored. This could be by using parenting apps or email to communicate so that it is easy to prove what is being said. If the harassment occurs in person, you might video these encounters or make sure that there is a witness to what happens.
There isn't any reason why your co-parenting relationship should be filled with stress and anxiety. You might need to look into having the type of plan you follow being changed to something that doesn't require this type of teamwork. A custody modification might be in order.