This only happens only on startup. It shows error unable to update calendar entry as the entry was not found. The entry seems to be the default US holiday dates. There are no errors for my entries that I create only the default US holiday dates.
Use Gmail and running most current emclient.
Sounds like that calendar event might be possibly corrupted somewhere locally or online.
I would suggest to first delete that Calendar entry via Google Calendar online, and then re-add it again via eM Client Google Calendar and see if you still get the error when you close & reopen eM Client…
Failing that i would click Menu / Backup to (make a backup) of the database, and then remove the entire Gmail account and re-add the account as new.