I got this same error when copying and pasting my husband's flight departure iCal item to the date that he was returning. I had just planned to change the word "departure" to "arrival" in the event to save time rewriting all the info. Boy was I bummed when this error showed up.
I looked at many many answers to this issue and was chagrined to see things about moving files, deleting files, backing up my calendar, reinstalling Yosemite (FYI: I'm on 10.10.3 on a MacBook Air).
Here are the key words associated with my error that will hopefully help people search and find this later: Error "400" to operation CalDAVWriteEntityQueueableOperation
Here's what I did that finally worked. Repeated the process because I accidentally triggered the server error again when I clicked on the offending iCal item that started the whole mess (it was there, so I thought I'd edit it).
I am going to speak from the viewpoint of a non-technical person (that's me, kind of. I'm a technical marketing copywriter):
- With iCal open, and the popup message with the server error and all the text below that's associated with the "offending" iCal item, do a select all (Command+A). This lets you select even the text below the bottom of your screen... important text to select).
- Copy and paste the entire message into a word doc.
- At the very bottom of the message, you'll see that it has a problem with one of your iCal calendars (e.g., I have these: work, home, kid calendar, school). For me, the problem was noted as being in my "work" calendar. Makes sense, as this is my default calendar, and I have to assign a different calendar if I want to add an iCal event to my home or kid calendar... you get the gist).
- Force quit iCal to close it (if you don't know how to force quit, you do it by pressing alt+command+esc, selecting iCal from the list of open apps that appears in the popup, and clicking the Force Quit button).
- Open System Preferences > Internet Accounts > iCloud.
- Select iCloud in the left side list. Things associated with iCloud show up to the right.
- In the list to the right, uncheck the Calendars checkbox. (I also unchecked reminders and contacts just in case).
- Reopen iCal. You'll see that all your iCloud calendars are gone. You'll still see calendars under "Other", which include for me US holidays and Birthdays.
- If it's not still open open, repeat step 5 to open the iCloud window in which you unchecked "Calendar." Sorry I don't know the official name for that window.
BEFORE YOU DO ANYTHING ELSE, READ STEPS 10 THROUGH 12 BECAUSE YOU HAVE AROUND 5 SECONDS TO COMPLETE THOSE STEPS BEFORE THE ERROR MESSAGE POPS UP AGAIN AND YOU HAVE TO REPEAT STEPS 4-9.
- Before the server error message pops up again, you need to do these things:
- Click the Calendar checkbox to turn it back on.
- Quickly uncheck the box next to the calendar that has the offending item.
PHEW. NOW YOU CAN RELAX.
- Click once on the name for that same calendar (the one you turned off). Then click it again to be able to rename it. Rename it with a different name. I renamed mine "Work2".
- Click the checkbox next to the renamed calendar. It turns on all the items associated with that calendar and the error message does not pop up any more. Unless, of course, you select it and spend time trying to do something with it.
ONCE AGAIN, DO THIS QUICKLY... BEFORE ERROR MESSAGE POPS UP.
- Quickly select the offending iCal item that you were creating when your issues started and hit delete.
Problem solved for me! I hope it works for you!
BTW, no loss of data, calendar items, etc occurs by just switching off your iCloud Calendars. I was unable to export or backup my calendars (that option was grayed out for me), so this resolution seemed like my only option other than manually writing down all my calendar items.