Dear Rad community, thanks for your  instructions and comments on changing the fuse.
My story is not new: the battery key touched the charging port, a spark came, the battery was not charging.
Radpower support sent out an offer for a new battery for 650 euros including shipping to Germany (I anyway could not complete an order as when I clicked on the invoice, it said ?sold out?!).
Two reputable bike stores in our city (we?re in Bavaria) refused to help saying they didn?t really know about RadPower bikes (although RadPower bikes are widely used by Flink, a major company to deliver groceries)?.
Luckily I have found this forum and now we?re back on track!