Getting an internal error when starting test suite in firefox headless (katalon studion 6.3.3-2ae238334

I have a test suite which has run many times successfully, this morning I tried to run it again (firefox, headless as I normally run it) and got the following error. Tried restarting katalon and updating the webdriver - which made no difference. Note: I can run the individual test cases ok

‘An internal error occurred during: “Launching the test suite…”.