How many fuses do you have on the + side ideally there should be one on each end of cable connecting batts together and one then on feed to EBL. In my experience German manufacturers fit Maxi blade fuses to battery wiring not as shown in your photo. Was the second battery a UK fit?
If the battery was tested with a discharge tester and the batteries were not disconnected that would certainly blow the fuse. A 30a fuse should be ample unless you are putting a very heavy load on them.