I was under impression that AMD hardware allows just one video encoding session and prevents from having multiple side by side. This has been the consistent behavior I was seeing and I was always wondering why it had to be that tight.
To my surprise, the actual limitation is higher and, in particular, is sixteen (16!) sessions runnable in parallel. In particular, with the GPU in my dev box…
The problem has been a bug in AMD driver and/or AMD AMF runtime, which triggered an exception when in low latency mode. With this bug it is indeed just one session at a time. Even though the bug has been present for literally years, it is good that AMD engineers do respond on github and this results in problem identification, workaround and I hope resolution as well.
The good thing is that just 2+ low latency sessions are not allowed. Multiple regular sessions and zero or one low latency, up to 16 in total, is still fine. That is, a fallback to non low latency session is a possible workaround.
UPDATE: This is a hardware limitation for AMD Polaris Architecture: only one process can own low latency hardware queue. Newer GPUs are not affected.