I have just changed my iCloud account and am now having serious problems with Calendar on my Macbook Pro. Any ideas?

Question

I decided to start using iCloud etc but discovered that my Apple ID doesn't actually exist (I was literally told this over the phone my an Apple Support worker) which is weird, as my email address for this Apple ID still exists. This is the first time I'd had any problems of this sort as I'd never had to use my Apple ID before. So anyway, I signed out of iCloud (i.e. signed out of my non-existant account) and signed in with my other, working, iCloud account which I've had absolutely no problems with whatsoever. All well and good.
Before signing out of/deleting my old iCloud account I moved all my events in Calendar from "iCloud" to "On My Mac" so that I wouldn't lose the information when switching to my new account. No problems with this. However now that I am signed in on my new account, every time I open Calendar I receive the following two error messages:
"Couldn't move your calendars to iCloud because an error occurred.
Make sure you're connected to the Internet, and then try again."
"The server responded with an error.
Access to account "iCloud" is not permitted.
The server responded:
"403"
to operation
CalDAVMigrateToServerQueueableOperation."
Note that with regards to the first one I am definitely connected to the internet. I have tried restarting my Macbook, have tried enabling/disabling Calendar in iCloud, nothing has worked. I always get these error messages. I have two options, "Try Again Now" or "Try Again Later", for the first message and "Try Again" or "OK" for the second. Pressing anything just results in the error messages just coming up again in a cyclic manner, after briefly trying to "Move calendars to server account". Sometimes a pop up also appears where it says "Upgrading calendars" with a progress bar but this freezes/never goes anywhere, and regardless of the outcome, I cannot access my information in Calendar. Note that behind these error messages etc I can see my events in Calendar but I cannot access the detailed information contained within them nor can I add any new information or essentially do anything in Calender. Sometimes Calendar crashes and I get a crash report.
Sorry for this dense block of information but I have no idea what's going on so I'm just trying to give a clear account of the problems I'm facing. I really hope someone will be able to help me with this and I thank you in advance.
Sam

Answer 1

Don't worry I've sorted it! I just had to turn off Reminders as well in iCloud. Calendar then worked fine, even when I turned Calendar and Reminders back on.