The HTTP2 protocol allows your client to set up connections in parallel. In the previous version (HTTP1.x) the requests were issued sequentially from the client.
Probably you are not able to reproduce the error, and you just observe them every now and then, or (better) you can reproduce them based on the topology of the network.
One source van be that your network is not capable of supporting HTTP2 end to end. Maybe you experience this when the client is behind the VPN of the corporate network. In that case, disable http2 (but the performance can drop 10-40%).
Or your server capacity is exhausted, not allowing servicing multiple streams.
You can install Wireshark or any packet sniffer on the client or the UAT server to see what the network is telling.
This one could be nasty. If disabling HTTP2 didn’t come with a performance degradation (most important is the PRD environment), this could solve the whole issue.
In the end, this is not a Mendix problem :-(