I know that this post is over a year old, but I can not find any other reference to this error regarding the kolab calendar plugin. I am getting the exact error that is posted here and have been getting that error each time I try to install the calendar plugin. This time I used composer to install the calendar and all it's dependencies. Yet, still the same error, and like your error above many, if not all, of the ics file's records are written to the error log.
I should post a new topic, but since this was so exact I wanted to see if anyone can shed some light on this problem.
TIA for any help offered.