I had a similar problem and changed to CalDAV subscription method, which at the moment seems to work, but was unstable, so I re-registered my calendar.

Now my problem is that the following error message comes up each time I modify anything or synchronize: "An error occurred when writing to the calendar Gmail_CalDAV! Please see below for more information." "Error code?: MODIFICATION_FAILED"

This is irrelevant but very annoying!

Please check, and advise! Many thanks, Gergely

I have a similar problem using multiple privateemail accounts and google accounts.

After synchronising the calendar or saving a calendar event the meaningless error code MODIFICATION_FAILED is generated and calendar events are not synchronised with the online calendar.

This has been a problem since the 78.10 update.

My calendar is render useless now until the problem is resolved.

A quick resolution will be appreciated.