Cannot connect to a https backend while a load test is running

Question

I wrote a load test that in 300 threads calls a simple web service.
The situation is
test client at onprem using java 1.8 jax-ws -> public Internet -> Azure Standard Load Balancer -> backend Windows machine -> Apache Server listening on port 443 -> Apache mod AJP proxy -> Apache Tomcat.

The ws uses azure sql and the whole scenario simulates a big insert / update load to a table with size of more than 300 GB.

While the test is sucessfully running I cannot see a lot of ERORRS like 'Cannot connect' etc.
The jax-ws client uses Connection:Keep-Alive
At the LB Monitoring I can see approx 100 SYN count - so some keep alive connections were closed and new one were estabilished.
At the Vm side I can see 300 ESTABILISHED connections using netstat -ano. That correlates to number of load client test.

But If I want to download even a static page /index.html from onprem side, the connection is stucked and nothing is returned, unless I stopped the load test client.
I tried a browser or a curl command.

Why?
Where should I continue in a diagnostics?

The machine was alone, dedicated for the test - so no one else called it...