calendar entries not being accepted

I have been using emclient for a year or more without any major problems. However  I now cannot make an entry into calendar. I will type in the appointment etc and the time and the category and when I press enter the  newly created entry pops on screen for about half a second then disappears completely. Cannot make any entries at all at the moment.

Hello, what calendar service are you using with eM Client? What version of eM Client do you have currently installed on your computer, can you please check the exact release number in Help > About? Are you using any security software on your computer, that may be causing this issue?

Does it disappear immediately after you hit save or on next re-sycnhronization of the calendar?

Thank you,