I took a seconds look at your capture you posted. So 10.18.207.91 is the endpoint which sends a RAS ARQ (Admission Request). Then there is some SIP processing. Then we see call rejected. Then the DMA sends RAS ARJ (Admission Reject). I still think it is an issue between the DMA and RPAD, but I would need the DMA server.log file to determine what the root cause might be. You can send me a PM of the DMA log file or instructions on how to download it instead of posting it, if you prefer.
As a quick test, can you download either the RealPresense Desktop or Mobile application and try to register to the DMA via SIP? Although your GS endpoints are not registering SIP, DMA still has to be enabled for SIP to be able to process SIP messages.