Thanks for the answer and effort!
I became aware of the problem through this review:
https://www.digitec.ch/de...
The post also just included the link to the corresponding UBNT Forum Topic.
I haven't really read up on it, and don't have enough knowledge to do so. The article mentioned the 1.9.1 firmware.
By the way: What throughput rates do you achieve? Do you get close to 1Gbit/s?