SOAP-UI Mocker fails when getting busy!?!

A colleague of mine has the strange effect, that a SOAP-UI mocker always fails when it receives requests too fast in succession. I.e. if one sends requests slowly with a bit of pause in between, the mocker responds fine. If one sends them in rapid succession then later requests (typically starting with the 3rd or 4th) always fail and return some HTML response listing the available mocks instead - as if the request contained a wrong operation and the mocker couldn't find a matching service.

This happens even when sending requests from SOAP-UI to itself (i.e. sending SOAP-requests to a mock running in the same SOAP-UI instance): If one hits the send button slowly then the requests yield valid responses. If one clicks the send triangle a few times in rapid succession then there will be an error response with an HTML payload instead. The mock responses are all simple and static, i.e. no calculations nor fancy query matching nor dispatch scripts or whatever. 

He asked me for help since I am somewhat experienced with SOAP-UI but I have never observed anything like that on my own system (where SOAP-UI runs rock-solid - knocking on wood) that's why I am asking the community.

Any idea what could cause this? This behavior is really blocking him because he can't decently test our application if the mock always returns garbage when being used normally.

BTW: Both systems are Window 10 developer laptops (up-to-date patch levels) with the very same SOAP-UI version installed (v5.5.6).