I am also having this problem. I installed Version: 6.1.2
Build: 16363 on Windows 10. I tried removing the "optimization" folder but still receive the same error below. I checked folder permissions as well. Any assistance would be greatly appreciated.
Error: TypeError: _reactcss.handleHover is not a function (http://localhost:5601/bundles/kibana.bundle.js?v=16363:67)
[2132]/window.onerror@http://localhost:5601/bundles/commons.bundle.js?v=16363:45:8728