Thank you very much everyone.
I have decided to disassemble once more in a few days when my partner is back and then sand all contact areas as well as using an aluminium contact grease before putting it back together. Looking at the replies it seems very likely that it's predominantly the battery assembly itself. Especially as the rest of the system had not been disassembled.
Unfortunately we don't have a thermal camera, but I touched every contact point and there where no warm/hot connections.
I have decided to disassemble once more in a few days when my partner is back and then sand all contact areas as well as using an aluminium contact grease before putting it back together. Looking at the replies it seems very likely that it's predominantly the battery assembly itself. Especially as the rest of the system had not been disassembled.
Unfortunately we don't have a thermal camera, but I touched every contact point and there where no warm/hot connections.