Victron recommends to use 2x 50mm2 per connection on a 400A fuse when using a 12/3000. I cant read what is stamped into the lugs but it could be 50mm2. I highly doubt its 95mm2, so one would be definitely not enough.
In the error description for overload it also says to check the DC cable connections and its cross section, apart from the obvious AC overload.
Also “two batteries” isnt really saying much. Are they big enough, charged enough, in parallel or one is the starter battery of the car and the other the house battery?