Browser is Chrome Version (64bits) 39.0.2171.95 m and mxhero is 2.8.0. OS are Win 7 (64bits) and Win 8.1 (64 bits) Version 6.3.9600

Same happens with me. I believe google must have made some changes to gmail and somehow it probably affected MXHero. I tried most advices I could find on the internet: reloading browser, uninstalling and reinstalling mxhero, etc. but to no avail.