I've searched and googled this until I'm blue in the face but all I ever get is how to switch sync on in the settings, which is already done.
I have one Windows 7 computer (it refuses to upgrade to Windows 10 and sticks at 92% every time) and three Windows 10 computers (all upgraded from Windows 7 or Windows 8.1).

I use calendar in Office online on the Windows 7 computer and the Calendar that came with 10 on the others. At first they synced without problem but soon this stopped. Now they either do not sync (it doesn't matter which of the computers I put the appointment/event in) or they suddenly sync after a couple of days. All of the computers sign in with my Microsoft account so they should be synced anyway.

The only other thing is that when I open the calendar (especially on the Windows 7 pc) the following message appears for about 7 seconds in a dark blue box in the top right of the calendar:
"Unknown message type signal/force/abchsync" The Skype logo is also in this box and if I click on it a mini-Skype opens up to the right of the calendar with this error message shown as a Skype message. This happens whether or not Skype is loaded on any of the computers.
I've search high and low but can find no explanation of this message anywhere.

Can anyone please explain what this message means and where it comes from (What's Skype got to do with it!!!) and/or suggest any ways I might get the calendars to sync.

Many thanks for any help you can offer.