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I think the times I've had a mesh error considering borders it's because I've done something like meshed one area with small tetrahedra, meshed a nearby area with larger tetrahedra, and then attempted to mesh the area inbetween. It fails to mesh in that case because it can't grow from the fine mesh size to the large mesh size in the space I've given it.

Asking COMSOL to mesh multiple domains on its own in a single step may produce similar errors. If you're just letting COMSOL mesh the entire region with Free Tetrahedral mesh, then an error like this probably means you'll need to add some customized mesh steps to help guide the meshing algorithm.

If you can identify the problem region, specify a meshing step to mesh only that part first. Configure that meshing step and run only that step at first to make sure the mesh in that region looks reasonable and is sufficiently fine. If necessary, you can then expand outward with your meshing by specifying another meshing step to just mesh another nearby or surrounding domain. Eventually, you can use a final "Free Tetrahedral" mesh of relatively coarse size to mesh the remainder of your domain. It can be frustrating at times, but this general procedure of starting at the problem areas and expanding outward has worked for me in most cases.

For more specific guidance, you might want to post your model file.

Also, are you modeling in 3D or 2D? Based on your description, it sounds like you are working in 3D, but if the problem is effectively infinite along one axis, then it can be solved in 2D. 2D would make everything way easier.

Thank you for your answer.

I will try to mesh my model step by step with finer mesh on smaller areas and see if I still get an error.

I am modelling in 3D, because I am trying to evaluate heating along the cable with forced convection thats why I can't model it in 2D.

I will Keep you posted if I manage to solve the meshing Problem.