Posting here in case anyone else is bitten by this: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=226637 ... patch attached to report.
$ mbuffer -i [any file] > /dev/null
mbuffer: fatal: cannot allocate more than 32767 blocks.
This is a system dependent limit, depending on the maximum semaphore value.
Please choose a bigger block size