The current sync state is stored in a file org-caldav-SOMEID.el in
the /.emacs.d directory. You can change the location through the
variable ~org-caldav-save-directory. SOMEID directly depends on the
calendar id (it’s a snipped MD5).
If you sync your Org files across different machines and want to use
org-caldav on all of them, don’t forget to sync the org sync state,
too. Probably your best bet is to set org-caldav-save-directory to the
path you have your Org files in, so that it gets copied alongside with
them.