This usually happens when the Mendix runtime cannot bind to the configured port, even if it looks like 8090 is free.
First, verify whether any process is actually using the port.
On Windows:
netstat -ano | findstr :8090
If you get a PID, stop that process:
taskkill /PID <PID> /F
Also check if there is a previous Mendix runtime instance still running in the background. Sometimes Studio Pro closes, but the runtime process remains after a crash or forced shutdown.
If nothing is using the port, try:
As a quick workaround, you can also change the runtime port from Studio Pro:
App Settings → Configurations → Edit → Server → Runtime Port
and test with another port like 8091.
Kindly mark this as the accepted answer if it helps.