FIXED!!! (for me, anyway).
My system contains some components that were manually installed rather than via install scripts. I knew this was probably somehow causing the problems.
I stumbled onto
this article and tried mbadici's approach but it quickly became an endless cycle of trying to include some other file. then I got some PHP fatal errors related to missing interfaces which could not easily be solved by just including another file. a new approach was needed, but I liked the train of thought.
Anyway, to fix this issue I followed the installation instructions for sabre here:
http://sabre.io/dav/install/specifically, the line that says "
To use it, just unzip it and include the autoload.php file in the vendor/ directory."
So, I just had to add a line at the top of my "<roundcube_installation_path>/plugins/libcalendaring/libvcalendar.php" to included this autoload.php file and the problem was solved. yay