Hi Marco,
Thank you for the reply.
We managed to solve it as follow:
The thick inner lines where caused by the overlap of upper and lower lines for each row in the Table. If you wanted 1pt thick rows you need to change the thickness of the top and bottom to 0.5pt
The outer table lines cut the border lines of the inner table.
To counter this we made sure there is row and columns all around the inner table like in the image.
You can shrink the rows and columns to a very small size and still get the result.
Further we made the inner lines 0.5pt and the outer border 2.5pt
