The problem is fixed now in master. It was caused by an attempt to shutdown the SSL session (which requires sending data) in a closed file descriptor, which is handled different between OpenSSL (no thread error queued) and LibreSSL (badsectoracula was using LibreSSL, which queues a thread error), and later causes an assert to trigger as it was not expecting any error in the queue. See[1] for more details.

[1]: https://github.com/dillo-browser/dillo/pull/59

Thanks again for the report, these type of issues are hard to reproduce without using different environments.