Awesome suggestion mwrusnak!
It was indeed the issue that we're behind a reverse proxy (sucuri WAF). For some reason (???) the Sucuri IP was not on the WordFence white list anymore.
So the first fake google bot that tried to enter the site, triggered WordFence and locked out the Sucuri IP, thus blocking everyone, including myself.
After white listing the Sucuri IP again, all went back to normal. Thank you!!
Maurice