Current bitcoin-master on testnet, maxconnections=8 in the config file. Running in VM that doesn't have public IP and is not reachable from the Internet.

Wait until you have 8 peers.

telnet localhost 18333

It will result in:

Assertion failed: (nMaxInbound > 0), function AcceptConnection, file net.cpp, line 973.

First found in #8996, but can be reproduced on master.