java.lang.AssertionError
at java.base/sun.nio.ch.Poller.register(Poller.java:192)
at java.base/sun.nio.ch.Poller.poll(Poller.java:171)
at java.base/sun.nio.ch.Poller.poll(Poller.java:137)
at java.base/sun.nio.ch.DatagramChannelImpl.park(DatagramChannelImpl.java:494)
at java.base/sun.nio.ch.SelChImpl.park(SelChImpl.java:116)
at java.base/sun.nio.ch.DatagramChannelImpl.untrustedReceive(DatagramChannelImpl.java:646)
at java.base/sun.nio.ch.DatagramChannelImpl.receive(DatagramChannelImpl.java:599)

From the traces I managed to obtain after adding a debug message to the AssertionError, it seems that a terminated virtual thread is still registered with the poller at the time a runnable VirtualThread tries to register.