USW-Flex - Maximum POE Availability Exceeded

Hi, just bought a USW-Flex and a USW-Lite-8-POE.

The Flex is connected to the Lite 8 via POE+.

The flex shows me a maximum POE power budget of 20W. Connected to the Flex is a POE+ splitter that splits into 1x RJ45 and 1x 12V DC. If I connect the 12V DC to a low-power device (internet modem in my case; 12V 0.8A) it works perfectly fine. The Flex shows me around 5-6W of POE usage for that port.

However, if I connect a my router (FritzBox in my case) to it, the Flex immediately starts showing me "POE Availability Exceeded", even though the FritzBox is also maximum 12V 1A (also pulls about 6W in idle). It should be nowhere near the maximum of 20W.

I already tried switching cables, tried another POE splitter, tried reverting the firmware version of the Flex to 6.4.18, but no success.

I'm seeing a lot of posts with similar issues on the Flex from about 3-5 years ago, but couldn't find out if those were ever fixed by Ubiquiti.

Has anyone had a similar issue with the Flex?