Visited on July 20 and did a short charge with eCarUp app.
The charger is inside the paid parking for the Sportzentrum. There's a ticket gate.
There's no official free period, but I was there for less than 10min and didn't have to pay. So at least there shouldn't be any issue if you enter, cannot charge for any reason and decide to leave again.
I didn't see any closing hours. Since you can park up to 24h, I assume it's open around the clock.
The first 2 pictures are for the parking area that's outside the gate, included for reference. It seems to be the same price, except for the first 40 minutes being free outside, but not inside (or at least not announced as such!)