I am sorry to be blunt but I think you have been resisting the help already offered 
You are correct that the error is informative.
May I be further pedantic. I think your interpretation of the “website” may be flawed.
In an ideal world you might add a few tasks and events to your device. When the device syncs, the database on your device is copied to a similar database on the cloud. Where you are correct to say you have ownership.
Now you should be able to access the cloud from any device and see the Calendar and Tasks displayed. You should be able to add another item. It should be transferred to the database on the cloud.
Next time the device and cloud sync, we expect both views to be identical. Any sync that sends an error implies an inability to achieve sync. (Remember sync = perfect. No errors are permitted.)
Once I have a sync fail, I assume that I must do or delete something until I can achieve sync again.
Reading what you have reported, my feeling is that in some way you have created a contaminated database at the cloud end. It might possibly have occurred in Calendar.
However, a process of careful housekeeping may resolve it fairly painlessly.
PS I get the bit about personal info in the logs! Further support email address is given here: Getting Started on /e/
Did you search the error:
iCalendar objects must at least have a component of type VEVENT
There is a lot of info out there to describe the significance of VEVENT of which I was previously unaware! I just pick this (not fully related) answer as it tends to support my imagined start of the issue: Error when creating a calendar object: "A calendar object on a CalDAV server may only have components with the same UID" · Issue #1264 · sabre-io/dav · GitHub