I don’t think it’s the length of the path… /etc/letsencrypt/live/ tends to be owned by root, and not accessible to anyone else (which is generally desirable, as you want private keys to be actually private). A couple of options on how to solve this situation are detailed here: node.js - Let's encrypt SSL couldn't start by "Error: EACCES: permission denied, open '/etc/letsencrypt/live/domain.net/privkey.pem'" - Stack Overflow
Another option is to set up a terminating reverse proxy, so the certificate handling is done by the reverse proxy (eg. Apache2 or nginx), and not by Collabora Online.