The problem that you’re running into is something that’s inherent with tangent normal maps. The math behind these maps dictates that the normals are flipped when a UV seam is crossed. As a result, if you’re using a tangent map you need to be careful about where the seams are placed so that you can hide them as much as possible.
You can import a new version of your model into subdivision level 1 in order to update the UV mapping. One technique that works quite well is to use UV Master to get your starting layout, painting protect areas onto the parts of the model where these seams would be visible and attract areas onto the parts where they’re going to be less visible. (AO is a nice help with this as a first step, followed by manually painted additions!) After you have the starting UV’s, export level 1 to whatever app you wish to use to refine the UV’s. You can then import the model back into level 1 of your ZBrush model, which will update the mapping in ZBrush. You can then even use UV Master again to relax the UV’s before you finally proceed to create your normal map.
Also, consider how your model will be used. Is it going to be animated? If so, then you need a tangent map. But if not, use an object map instead by turning Tool>Normal Map>Tangent off before creating your map. Object-space maps use different math and aren’t affected by UV seams.
Another helpful trick is to create both a normal map and a displacement map for your model. This is done by dropping down a couple levels from the highest and creating your normal map. This will contain the high resolution details, only. Save your model, then delete the higher levels. Now go to level 1, restore your base mesh and create a displacement map. Because you deleted the top couple levels, this map will only contain the medium resolution details. The advantage to this workflow is that low and medium resolution details are where you’re most likely to see seam problems. Since you’re using a displacement map for this, that’s a non-issue. High resolution details don’t show up seams nearly as easily, so you can use a normal map without issue. This is especially true when the model is actually animated because the eye can’t track high resolution details as easily when the object is in motion. You will still naturally want to hide the seams as much as you can, but this dual map approach will greatly reduce any issues.
