Avoid full-body planks. modify by working at a kitchen counter—and only if you feel like you can fire the deepest layer of your core (aka, keeping the ab wall in and firm without "sucking in"). Once you feel comfortable with this and ready for more, you can start to increase the load by taking Planks on the back of a chair and eventually the floor.
Avoid abdominal crunches and full-body rolls. These require the ton layer of the abs to work like crazy. If the rest of the core is still a bit unstable and weak, this will push the ab wall up and out, therefore putting pressure on your linea alba. You can keep your head on the floor and focus on keeping your hips and spine still as you move your lower body.
Keep both legs in the air while on your back. This might be tough to do in the beginning. You can modify by placing one foot on the ground or placing the barre3 Core Ball under your seat. Also, keep your movement smaller and higher and only lower your legs more if you can keep the core engaged.
Through time, you might be able to go back to traditional ab work—however, listen closely to your body and be careful not to push it beyond its limits! Trust that other moves strengthen your core, too, and they can actually give you the results you are looking for!