Incase you haven’t fixed it yet
I can spot two possible issues:
in caldavio.cfg youre missing the letter c on the line for the username.
aldavio:muell:username=XXXXX(without @gmail.com)
in caldavPersonal.cfg
should be
caldavPersonal:usedCalendars=muell
Otherwise your setup looks like mine, and i have not had problems with updating caldav items, atleast so far 
oh, forgot to mention, you might want to check this thread out: https://community.openhab.org/t/how-to-remove-caldav-reload-job/16163
Theres some config issues with the caldav binding and openhab 2, this thread helped me with a similar problem, might be helpful for you too.